More details have emerged about what will be in the federal budget, with billions of dollars earmarked for roads and rail over the next decade and a small tax cut hinted from July this year.

Key points: $24 billion over a decade to be announced for road and rail

$24 billion over a decade to be announced for road and rail Small tax cut expected for low- and middle-income earners from July

Small tax cut expected for low- and middle-income earners from July Budget could be back in surplus a year earlier than forecast

A $24 billion infrastructure package will deliver funding for key road and rail projects that the Government hopes will "bust congestion" and reduce the road toll.

The budget is also set to include tax cuts for people on less than $87,000 a year from this July.

There are signs the Government is also likely to announce a budget surplus for 2019-20, which is a year earlier than forecast.

Finance Minister Mathias Cormann said the Government wanted to ensure the budget returns to surplus as quickly as possible.

Revenue has recently surged and some economists are warning it would be more prudent for the Government to use that to start paying back debt rather than offering small tax cuts.

Senator Cormann defended the decision to cut tax for low and middle-income earners instead of using the money to pay back debt.

He said the economy would perform "less well" without the tax cut.

He has not revealed the size of the tax cuts to be announced tomorrow, but Treasurer Scott Morrison said they would not be "mammoth".

An income tax break for high-income earners is tipped to be revealed in the budget, but it would not come into effect for several years.

Labor's finance spokesman Jim Chalmers contrasted the income tax cut with the Government's existing promise to cut company tax.

"The highest priority for the Government is not the working people of Australia, the highest priority for the Government are the multinationals and the big banks, that much is clear already" he said.

"I don't think any income tax cut in the budget tomorrow night can properly make up for the damage that has been done to families in this country over the last five years."

He said the income tax cuts appeared to be based on a temporary spike in revenue and optimistic projections about wages growth.

Projects aiming to bust congestion and reduce road toll

The biggest infrastructure project is the rail link to connect Melbourne's airport to the city, which was announced earlier this year and will cost $5 billion.

There is also $400 million to shift freight from roads to rail, which Prime Minister Malcolm Turnbull said was to "bust congestion".

As well as reducing pressure on clogged roads, some of the spending is aimed at reducing the road toll.

Nearly $1 billion will be spent on building a Coffs Harbour bypass on the Pacific Highway.

"That is going to save lives," Infrastructure Minister and Deputy Prime Minister Michael McCormack said.

Mr Turnbull said there would be infrastructure spending right across the nation.

"Nobody has missed out," he said.

The federal budget comes amid a potentially tumultuous week in politics, with the High Court due to announce on Wednesday whether Labor senator Katy Gallagher is ineligible to sit in the Parliament because of dual citizenship.

That decision could mean up to four further by-elections.