The electrification of the Gawler rail line and the completion of more stages of the South Road upgrade are among projects earmarked for more than $1.5 billion worth of infrastructure funding in tomorrow's federal budget.

The north-south corridor will get $1.2 billion, $220 million will go towards electrifying the Gawler rail line and $160 million has been allocated for the duplication of the Joy Baluch Bridge in Port Augusta.

Federal Urban Infrastructure Minister Paul Fletcher said there still needed to be some negotiations about what contributions the State Government would make to the projects.

"We'd expect there'll be a significant funding contribution in due course from the SA Government," Mr Fletcher said.

"What needs to happen now is planning for further stages of the north-south corridor, beyond Regency Road to Pym Street, but what we're committing is significant funding for future stages of the north-south corridor."

Parts of the north-south corridor are already completed or well underway, such as the Torrens to Torrens section of South Road and its Darlington upgrade.

An artist's impression of an electric train at Gawler railway station. ( ABC News )

Work on a 1.8-kilometre six-lane motorway from Pym Street to Regency Road will begin next year, with the State and Federal governments agreeing last week to split the $354 million cost on a 50-50 basis.

State Infrastructure Minister Stephan Knoll said the Federal Government would pay for 80 per cent of the Joy Baluch Bridge work.

"That's fantastic and in line with our expectation," Mr Knoll said.

"The Gawler line electrification — this Federal Government contribution is enough to complete the corridor without any real extra state government money, and we look forward to negotiating on the remaining sections of the north-south corridor, as well as other projects."

Opposition transport spokesman Tom Koutsantonis said he was concerned the amount of planning still required suggested the funding would not be in the forward estimates.

"Why have they allocated funding if the business case isn't ready?" Mr Koutsantonis said.

"How do they know what it's going to cost if they haven't done the business case and they haven't done the planning — this is just stalling tactics.

"If the money's in the forward estimates of the budget tomorrow night — that is, the Government is actually spending the money over the next four years to do South Road — this is great news. If the money's not in the forward estimates, we've been conned."

A 2015 scoping report completed by the State Government forecast a $2.5 billion price tag to complete the non-stop road between the River Torrens and Anzac Highway, and a further $2 billion for Anzac Highway to Tonsley.

Almost $1 billion of the total $4.5 billion price tag (in 2014 dollars) would be spent acquiring 466 properties.