But the prime minister is facing a tough sell, with wages growth falling short of budget expectations for several years.

The government has forecast a $7.1 billion surplus at the end of 2019/20, with strong commodity prices helping to boost its revenue in the current financial year.

Wages growth - which is among key parameters on which figures such as a surplus are based - is expected to pick up to 2.75 per cent in 2019/20 and 3.25 per cent in the following year.

Those forecasts are more modest than they were in the last budget update in December, when the government was expecting 3 per cent wage growth in the coming financial year.

Wages grew by 2.3 per cent across Australia in the year to December, Australian Bureau of Statistics figures show.

Shadow treasurer Chris Bowen says the forecasts remain "optimistic" or even "heroic" as the government lacks a plan to get wages rising more quickly, and show how tenuous the rest of the budget is.

"At least in part, this budget surplus is based on a wish and a prayer," he told reporters in Canberra on Wednesday.

Mr Morrison says wages will go up through coalition policies promoting business growth.

"The wages growth is going to come from businesses continuing to expand ... because of the low tax environment we're creating for them," he told ABC's Radio National.

Treasurer Josh Frydenberg has also stressed the significance of the government's policies aimed at improving productivity, a vital factor to long-term wage growth.

Such policies include spending on infrastructure and skills training, he says.

"When it comes to the economic outlook, we need to boost the productive capacity of the economy," he told the National Press Club in Canberra.

But the government has also pared back expectations for economic growth, forecasting 2.75 per cent growth for 2019/20, down from its earlier expectation of three per cent.

Many economists believe that is still somewhat optimistic, with NAB anticipating growth of about 2.25 per cent in the year, and AMP expecting 2.3 per cent.

That comes as Australia's trade surplus unexpectedly improving in February from $4.35 billion to $4.8 billion, according to the ABS.

Exports edged 0.2 per cent higher in the month, again supported by high iron ore prices and gas exports, while imports were down one per cent, thanks to lower fuel exports.

Australian Associated Press