Further funding for South Australia's largest infrastructure project will be included in Tuesday's budget as the state government continues to grapple with a big cut to revenue.
The final section of Adelaide's north-south road corridor will cost more than $5 billion with the bill to be shared by SA and the commonwealth.
On Tuesday, Treasurer Rob Lucas will deliver $252 million over the forwards to cover planning and early works for the final 10.5 kilometre stretch of the 78-kilometre corridor from the River Torrens to Darlington.
"This last piece of the north-south corridor puzzle will be the single biggest infrastructure project in South Australia's history," Premier Steven Marshall said on Sunday.
"Once complete, the north-south corridor will provide an economic boost for the state, increase freight productivity and cut travel times and improve safety for all motorists.
"This builds on our record $1.115 billion investment in regional road and infrastructure, as well as the nine new intersection upgrades we'll be building across metropolitan Adelaide."
But there remains no firm completion date for the project with the government considering three options for the work, two of which include tunnelling.
The funds for planning also come amid problems for Mr Lucas as he contends with a $500 million shortfall in GST returns for the coming year which threatens to derail promises to deliver budget surpluses.
The treasurer has already moved to increase a range of taxes and charges with some, including fines for certain motoring offences and liquor licensing fees, hit with big increases.
The Labor opposition has zeroed in on a range of new measures, criticising the government for slugging those who can least afford it.
On Sunday, Labor targeted a proposed $50 increase in the emergency ambulance call-out fees which it said came on top of big increases in hospital car parking charges.
"This is the latest cash grab by the Liberal government, reaching into the pockets of South Australians at what can be their darkest hour," health spokesman Chris Picton said.
Australian Associated Press