Morrison won't confirm surplus timing

Treasurer Scott Morrison has declined to confirm whether the government is sticking to its prediction for a budget surplus in 2020/21.

Instead he stated the obvious - Australia's finances will be back in the black when expenditure is less than revenue.

"You don't have to be a rocket scientist to work that out," Mr Morrison said in response to a Labor question during parliamentary question time on Monday.

Deloitte Access Economics predicts a boost to national income from a sharp rise in coal prices will only partly offset a deterioration in tax revenues on the back of record low wage growth and job creation undershooting official forecasts.

It's economist and ardent budget watcher Chris Richardson expects the budget deficit to be $40.5 billion in 2016/17 rather than the $37.1 billion forecast in the May budget.

Shadow treasurer Chris Bowen asked the treasurer whether the budget would be back in balance in 2021 if its $50 billion worth of business tax cuts were dropped

"I don't intend to engage in the hypotheticals of the member opposite," Mr Morrison replied.

"I ask him what does he think the impact on the budget is going to be if they continue in their approach of blocking $19 billion in budget savings measures?"