To be eligible for the JobKeeper Payments from 28 September 2020, businesses will be required to meet a decline in GST turnover for each period of the extension. From 28 September 2020, businesses seeking to claim the JobKeeper payments will be required to demonstrate that they have suffered a decline in turnover using the actual GST turnover, not projected GST turnover.
Consequently, from 28 September 2020 to 3 January 2021 businesses will need to demonstrate the business declined in the September 2020 quarter relative to the comparable 2019 period. And from 4 January to 28 March 2021businesses will need to demonstrate the business declined in the December 2020 quarter relative to the comparable 2019 period.
https://treasury.gov.au/sites/default/files/2020-08/Fact_sheet-JobKeeper_Payment_extension.pdf