General Advice Disclaimer
As at the time of writing, very limited commentary is available from the ATO or elsewhere regarding the JobKeeper payment. In all cases I've made my best effort at accurately interpreting the available information, however the information contained on this page may include misinformation, and is offered without warranty of any kind. In all cases, you should seek specific advice from an appropriately qualified person.
One of the eligibility requirements for the JobKeeper payment is that the employer passes a Decline in Turnover Test.
The test is simply whether a comparisson between a month or quarter in the 2020 calendar year, with the same period for a prior year demonstrates a reduction of greater than 30% of GST Turnover.
Once you pass the test at any point in time, you will satisfy this requirement for the remainder of the JobKeeper fortnights. You do not need to satisfy the test each fortnight or month.
There is a modified definition of GST Turnover for this case. There are many adjustments contained in the rules. Most notably is that for the purposes of the test we assume that the business is registered for GST.
(9) For the purposes of subsection (8), in determining whether the entity made a supply (within the meaning of the GST Act) that is a taxable supply: (a) assume that the entity is registered (within the meaning of that Act); and (b) assume that the supply is neither GST-free (within the meaning of that Act) nor input taxed (within the meaning of that Act); and (c) for an entity carrying on business solely in the external Territories—assume that the external Territories are part of the indirect tax zone (within the meaning of that Act).
Coronavirus Economic Response Package (Payments and Benefits) Rules 2020 Div3.s12.p9
The test period may be either a month or quarter. There does not appear to be any requirement in the rules or explanatory notes that you need to use a test period that matches your BAS cycle. For example, if you lodge an activity statement quarterly, there does not appear to be a requirement to apply the turnover test to a whole quarter.
The Decline in Turnover Test can be applied to a projection, you do not need to wait until the end of the month to determine "actuals".
For example, suppose that on 1 May 2020, you prepare a projection of your income for the month of May and determine that your turnover is likely to show a reduction of 30% when compared to your income for May 2019. You could register for JobKeeper payments immediately, and would receive JobKeeper payments for May. Alternatively, if you wait until the end of May, to perform the Decline in Turnover Test, you may still become eligible, but would not be entitled to payments for the month of May.