Treasurer Josh Frydenberg says the changes, along with more Victorian companies joining the scheme, will add another $15.6 billion to the cost of the wage subsidy program.

Previously, businesses and not-for-profits hoping to receive JobKeeper from September 28 to January 3 would have to show a significant fall in turnover in both the June and September quarters.

But under the nationwide changes, they will just have to show turnover has significantly fallen for the September quarter, compared to the same period last year.

Businesses and not-for-profits will have to prove their eligibility again in January, but it will be based off the December quarter rather than the two previous quarters as well.

Mr Frydenberg says Melbourne's stage four restrictions will have a significant impact on the Victorian and national economy.

"We will continue to do what is necessary to cushion the blow and help Australians get to the other side," he said on Friday.

Staff who were employed as of July 1 will now be able to access the program.

The payment is currently $1500 a fortnight but will reduce to $1200 for full- time employees from October to December.

It will then reduce again to $1000 until March.

Prime Minister Scott Morrison says the tighter coronavirus restrictions in Victoria will lead to as many as 400,000 people losing their job or seeing their hours reduced to zero, and a 10 per cent national jobless rate by year's end.

Mr Morrison said the revised Treasury forecast was for a reduction in real gross domestic product in the September quarter of up to $12 billion.