From the start of the NEM (13th December 1998) through until 30th September 2021, the NEM has operated with 5 minute dispatch and 30-minute settlement (three months later than initially planned).
As a result of a rule change proposal submitted by Sun Metals Corporation (large energy user in North Queensland that has been spot exposed and hence providing Demand Response into the market for many years), the AEMC conducted a Rule Change process discussed here.
On 28 November 2017 the AEMC made a final rule to change the settlement period for the electricity spot price from 30 minutes to five minutes, starting in 2021. The AEMC notes:
Five minute settlement provides a better price signal for investment in fast response technologies, such as batteries, new generation gas peaker plants and demand response.
Some articles posted on WattClarity provide some context to either:
1) The 5/30 Issue (linked here), as the underlying reason for the change; and
2) Five Minute Settlement itself (linked here).