The South Australian Liberals have formed an unlikely alliance with the Greens and Family First to provide volunteer firefighters the same access to cancer compensation as career firefighters.

The State Government's scheme introduced last year means MFS firefighters who contract certain forms of cancer are automatically presumed to have got the disease through their work.

The same cover extends only to Country Fire Service (CFS) volunteers who have attended 175 fires over a five-year period.

A total of 12 cancers are on the list, each with an associated period of service before the compensation kicks in.

Opposition Leader Steven Marshall says he cannot yet say how much it would cost to broaden the scheme, but disputes figures provided by the Government.

"They have got no costing credibility whatsoever. At the moment they're out there saying this is going to cost up to $23 million per year," he said.

"We reject that. It's not consistent with any of the evidence from other jurisdictions around the world. We're getting our own separate actuarial advice.

"It's not like you fight one fire and then all of a sudden you are provided with the opportunity where you have automatic access to the compensation."

There are currently about 13,000 CFS volunteers in South Australia and an actuary's report suggested the likely annual cost of equal cover could be much less than the 'worst-case' estimate of $90 million.

But the report also found it would cost about $25 million per year to extend the compensation to volunteers.

Greens MP Tammy Franks says she supports eligibility requirements but not the additional criteria for CFS volunteers.

"We're asking simply for that first hurdle. It's a fair hurdle. It's based on the science. [We want] equal treatment for MFS and CFS," she said.

Premier Jay Weatherill says the Opposition's position is economically irresponsible.

"They want it both ways. They want to say something kind to the volunteer firefighters but then actually say it doesn't cost anything," he said.

"The truth is we've had their proposition costed and it costs $90 million.

"It's irresponsible to give a presumption in favour of a work-related cancer for instance on day one of someone joining up the CFS volunteers.

"It would expose the state to a multi-million dollar claim."

The Government has previously said it will review the figures if it wins the election.