Under the National Electricity Rules, Scheduled and Semi-Scheduled units need to have submitted ‘Daily Bids’ (sometimes called ‘Initial Bids’) for the following Market Day by 12:30 the day before (this is sometimes talked about as ‘Gate Closure’).

At this time, the unit’s 10 x Bid Price Bands are fixed for the following market day, but the volume offered in each of those bands for all Dispatch Intervals (both now, and into the future of predispatch) can vary – using a Rebidding process.

Through the history of the NEM, there has been several different permutations of Rebidding Guidelines issued by the AER (and its predecessors).  These are summarised in the following table:

1st Oct 2021 – future

In conjunction with the introduction of 5 Minute Settlement, the AER introduced updated Rebidding Guidelines.

As discussed on this AER page, this process was initiated on 18th September 2019, with a draft decision on that day and a final decision on 27th November 2019.

The principle documents are the two that follow:
27th November 2019 = this “Rebidding and Technical Parameters Guideline – Final Guideline November 2019” , which became effective from 1st Oct 2021.
27th November 2019 = this “Rebidding and Technical Parameters Guideline – Final Decision” published to explain the reasoning for the Final Guideline.

In conjunction with these AER Guidelines, the AEMO also published this document “Format and Validation for Energy, FCAS, and MNSP Bids and Offers” in January 2020.  This was accompanied by the upgrade of the MMS Data Model to version 5.0 to support the implementation of Five Minute Settlement.

28th Feb 2017 – 30th Sept 2021

In February 2017 the AER published a revised version of the Guideline to replace the previous (December 2009) version.  The release of the revised Guideline (and accompanying final decision) marked the end of a consultation process which began with a consultation paper in September 2016 followed by a draft decision in December 2016.

The revised Guideline became effective from 28 February 2017.

Documentation is available from the AER website here – including the “Rebidding and Technical Parameters Guideline – Feb 2017”.

Note that this guideline provided some updates to the requirements established in 2009 – however it was the guideline of 2009 that provided a large step in the formalization of guidelines for rebidding in the NEM 

1st Dec 2009 – 27th Feb 2017

On 25 September 2009, the AER published the preceding version of the final Rebidding and Technical Parameters Guideline.
(a)  This version outlined the detail that needed to be be contained in a rebid reason submitted to the Australian Energy Market Operator (AEMO).
(b)  In addition, it covered several related areas associated with the bidding and rebidding of technical parameters.The AER recognised that compliance with the new Guideline would require a change to the business practices of many participants.

The revised Guideline became effective from 1st December 2009.

Documentation is available from the AER website here – including the “Rebidding and Technical Parameters Guideline – Dec 2009”.

It’s also worth noting that, in this period (on 10th December 2015) the AEMC made a final rule to enhance the arrangements that govern generator’s offers in the wholesale electricity market.  The final rule amended the relevant provisions in the National Electricity Rules as follows:
(a)  The current requirement that offers be made in good faith will be replaced by a prohibition against making false or misleading offers.
(b)  Any variations to offers will need to be made as soon as practicable.
(c)  A requirement to preserve a contemporaneous record of the circumstances surrounding late rebids will be introduced.These changes were assessed in the AEMC project known as ‘Bidding in Good Faith’.

13th December 1998 – 30th Nov 2009

The initial 11 years of NEM operation incorporated the operation of the National Electricity Code Administrator (NECA) as the predecessor to the Australian Energy Regulator (which was established in July 2005).

With respect to discussion about Bids and Rebids on WattClarity and elsewhere, readers might be interested in the following:


Category of Rebid

In the 2017 Guidelines, the AER required rebids to be attributed to one of the following categories:

Category “P” for Plant Reasons
Category “A” for AEMO Forecast or Dispatch Change
Category “F” for Financial/Commercial Reasons
Category “E” to Correct an Error

In the 2019 version of the Guidelines, the AER clarified that the submission of Rebid Category was recommended, but not mandatory.


Form of Rebids –

‘Well Formed’ and

‘Not Well Formed’

It is the AER’s role to assess ‘Compliance’ of each Participant’s participation in the NEM … including with respect to the compliance of their rebidding practices.

Readers should be clear that nothing we do is intended to imply whether (or not) any particular Participant is Compliant with the Rules.

In the various iterations of the Generator Statistical Digest that we have released from 2019 onwards, we have found it useful to categorise bids as to whether they are ‘Well Formed’ or ‘Not Well Formed’:

Readers should be clear that the ‘Well Formed’ label:
1)  is one that we created with reference to some (but not all) aspects of the AER’s Rebidding Guidelines.
2)  It is not intended to imply Compliance,
3)  but rather is a fully automated method of parsing the exponentially increasing number of rebids being submitted (see ‘The rise of the auto-bidder) to provide a high-level guide that could be included in documents such as the GSD to help the reader understand, at a high level, the type of approach being taken by various Portfolios in bidding their Scheduled or Semi-Scheduled assets.

The method of determining ‘Well Formed’ or ‘Not Well Formed’ has varied through time, as different versions of the AER’s Rebidding Guidelines have been implemented – as outlined loosely in the following table:

1st Oct 2021 – future

Five Minute Settlement commenced on 1st October 2021 – but participants could submit bids in the new format (compatible with MMS v5.0) for several months prior to this time.  Each participant

For the GSD2021, the monthly aggregates of bids ‘Well Formed’ thus needed to straddle both pre-5MS (below) and post-5MS (here):

In the AER’s Final Decision in 2019, they clarified that:

1)   Some fields (‘eventTime’ and ‘reason’) were mandatory; whilst

2)  Other fields (‘category’, ‘awareTime’ and ‘decisionTime’), whilst not mandatory, were still highly recommended….:

‘as the most efficient means for relevant participants to provide, and the AER to receive, relevant information to be able to substantiate the reason for a rebid’
 (see p7/11).

With this context in mind, we consciously chose to automate a check for having all 5 fields filled in, within a rebid, for it to be categorised as ‘Well Formed’ within the GSD2021 (e.g. as a form of ‘best practice’ automated check).

Hence in many (but not all) of the cases in the GSD2021 where rebids have been classified as ‘Not Well Formed’, this has been due to the fact that the participant has chosen to submit the 2 mandatory fields, but not all of the 3 other recommended fields in the bid.

28th Feb 2017 – 30th Sept 2021

The AER’s February 2017 Rebidding Guidelines requested participants to compose their rebids in a specific format for submission.

HHMM {space} Category {space} DDD…D

In the Generator Report Card 2018 we wanted to parse all the rebids made by generators (particularly since this guideline came into effect) in order to understand how many were made in each Category, and also for each plant type.

To do this we coded an algorithm that would parse all the bids using a strict interpretation of the structure of the rebid above and were surprised to find that the vast majority of rebids were ‘Not Well Formed (Strict)’.

By loosening the algorithm (i.e. taking into account the common types of failure modes of bids that we saw) we were able to successfully parse the Category and Time from the majority of rebids – however there were still a sizeable number of rebids that did not even pass that test, and were classified as ‘Not Well Formed (Loose)’.

This was discussed in this article from February 2019, and in more detail in the GRC2018 – and also in the subsequent GSD2019 and GSD2020.

Early years

This type of logic is not possible to apply at all in the early years of NEM operations.

Each annual issue of the Generator Statistical Digest is described separately on its own page.


Number of Rebids

In the Generator Report Card 2018 we trended the number of rebids over time (by plant type and category).

This was discussed in this article from February 2019, and in more detail in the GRC2018.