The budget surplus is almost certainly gone as economists warn the Morrison government's run of good luck has finally ended with the coronavirus and bushfires punching holes in revenue and forcing up spending.

A former Reserve Bank board member said the government's plan for a surplus was already unsustainable because it depended on a string of luck, while economists said Treasurer Josh Frydenberg faced an uphill battle to deliver a promised $5 billion surplus for the 2019-20 financial year.

The government, which highlighted its promised surplus during last year's election campaign, has sought to play down expectations in the past fortnight as the extent of the coronavirus outbreak has grown.

Australia is set to extend its coronavirus travel ban beyond Saturday in a move expected to hammer the education sector and block 100,000 international students from entering the country in time for the start of semester, government sources confirmed.