Are you aware the measurement method of GST Turnover has been changed by the ATO?
The JobKeeper scheme has been extended from 28 September 2020 until 28 March 2021 and the extension periods are:

Extension 1 -  from 28th September 2020 to 3rd January 2021
Extension 2 – from 4th January 2021 to 28th March 2021.
It should be noted that there were some big changes in the Decline in Turnover Test in the JobKeeper Extension – and it appears that it will affect the JobKeeper extension eligibility status for many businesses.
One of the biggest changes is that the business will now need to satisfy the “actual decline in turnover test”. The main changes from the original decline in turnover test are:
  1. The test will be no longer be based on months, but on quarters;
  2. The decline in turnover will now be based on Actual Turnover rather than Projected Turnover;
  3. The accounting method to determine “GST turnover” will be based on the GST method the business is registered for as per its Business Activity Statement (BAS).  In most cases, it will be based on the figure in G1 on the BAS for the current period (ie September 2020 quarter) and the comparison period (ie September 2019 quarter);
Under this method, a typical small to medium business where it would have registered GST on a cash basis (at least for the last 12 months), must measure it's GST turnover on a cash basis and is most likely required to measure its JobKeeper Extension eligibility by:
  1. Measure its GST turnover for the September 2020 quarter on a cash basis;
  2. Measure its GST turnover for the September 2019 quarter on a cash basis;
  3. Compare the two and determine whether the decline (if any) was 30% or more.  If the decline was less than 30% then the test is not satisfied
Under most circumstances, the business can also measure its actual decline in turnover by simply comparing the G1 figure on the BAS for the two periods mentioned above. 
The alternative test has largely remained the same but it needs to be tested based on quarters and its measurement method is also determined by the  BAS reporting method of the business.  There is a test for each of the following:  
Please note on the Business Portal website under the JobKeeper section the ATO has already provided guidance and a calculator on how a business should check its decline in turnover – simply by comparing the G1 figure in the September 2019 quarter BAS against the September 2020 quarter BAS. 
We strongly recommend all clients to contact their client relationship manager or the partner involved with their business to review their businesses’ eligibility for the JobKeeper extension should they have any doubts.
Detail information can be found in the following government websites:

Legislative Instruments

Explanatory Statements

Details on the ATO website