The federal government has promised an extra $1.5 billion to complete Adelaide's north-south road corridor, with the details to be included in Tuesday's budget.
The commonwealth has already pledged $1.2 billion to complete the South Road upgrade with the extra cash and money from the state government putting a total of $5.4 billion on the table for the key infrastructure project.
South Australian Liberal Premier Steven Marshall says once the final 10.5km section is complete, Adelaide will have a 78km road corridor running from Old Noarlunga in the south to Gawler, north of the city.
It remains to be seen how much of the federal cash will be allocated across the budget forward estimates, with most of the money expected to be included in the commonwealth's existing 10-year infrastructure fund.
But this should not be a major concern for the state government, with work on the final section not expected to start for several years and SA still exploring its options for the project including the use of tunnels and overpasses.
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Some cash will likely be allocated sooner for land acquisitions.
Mr Marshall said the final section of the north-south corridor would be the biggest single infrastructure project in SA's history.
"This project will be the major route for freight and commuters travelling north and south and will slash travel times and boost economic productivity," the premier said on Monday.
"Importantly, this project will underpin thousands of construction jobs for almost the next decade."
Prime Minister Scott Morrison said his SA colleagues had made sure the North-South Corridor was a budget priority.
"This investment is about getting Australians home sooner and safer," he said.
"I want people to spend less time in traffic and more time doing what's important to them."
