Liberal leader Steven Marshall has come under fire for his party's response to the controversy engulfing Mount Gambier MP Troy Bell, who is facing criminal charges and has resigned from the party after an anti-corruption investigation.

It was revealed today Bell's decision, which has thrown Liberal preselection in the seat in South Australia's south-east into turmoil, followed a probe by the Independent Commissioner Against Corruption (ICAC) Bruce Lander.

Bell will appear in court on Tuesday to face charges including 20 counts of theft that allegedly occurred over four years, before he became an MP.

"It will be alleged that the 43-year-old man from Mount Gambier dishonestly dealt with a substantial amount of public money," Mr Lander said in a statement.

"It will be further alleged that the public officer used documents known to be false, with the intention of claiming a benefit for himself."

The former teacher ran an Independent Learning Centre in Mount Gambier before his election in 2014, but is maintaining his innocence and vowing to fight the charges.

Labor is calling on Bell to immediately resign from Parliament and force a by-election, but he has committed to remaining as an independent.

He today spoke publicly about the matter for the first time, and is refusing to rule out contesting the next election.

"That's too early to determine. This has taken a very heavy toll on my family and my staff," he said.

"I'm certainly not ruling it out, but at this stage it is too early to tell. I'm still in a state of shock and disbelief."

The revelations have forced the Liberals to begin a search for a new candidate to contest the March 2018 election.

The party is expected to face significant challenges from other independents, including the Nick Xenophon Team, which is battling internal dramas of its own.

Liberal preselection for the seat closed earlier this year with Bell unopposed, but has now reopened until September 8.

A senior south-east Liberal, who did not want to be named, has told the ABC that the party "is now unprepared in what should be a safe seat".

"This seat has a history of going to independents — Rory McEwen and Don Pegler — and now we face that challenge again — to fend off independents and a Xenophon candidate," he said.

"To be back in that position is astonishing."

Marshall won't pressure Bell to quit Parliament

Mr Marshall has defended his response to the controversy, after admitting he first learned of the issue on Sunday when Bell called him.

"We've moved as quickly as we possibly could," Mr Marshall said.

"We first spoke about this issue on Sunday. He made it clear that the details would be forthcoming on Tuesday, which they were."

Mr Marshall tells the media the Liberals acted swiftly on Bell's resignation. ( ABC News: Nick Harmsen )

Mr Marshall said he again spoke with Bell on Wednesday when the MP offered to resign from the party.

"I asked him to put it in writing, that was received on Thursday," he said.

"We moved very swiftly and 4:00 yesterday afternoon the state executive confirmed that they would be accepting his resignation.

"They've opened nominations for Mount Gambier. It's a very important seat for us in the lead-up to the next election and we're now in the preselection process."

Premier Jay Weatherill has called on Bell to quit Parliament, and described Mr Marshall's refusal to demand the same as a leadership failure.

"When Troy Bell resigns from the Liberal Party because he's facing major charges they say it's appropriate that he remains in the South Australian Parliament," he said.

"This is rank hypocrisy. It's motivated by one thing and one thing alone and that is what's good for the Liberal Party, not what the right thing to do is."

But Mr Marshall today resisted that pressure and refused to intervene, and said he would not be urging Bell to quit and force a by-election.

"That's a matter for him," Mr Marshall said.

"He insists that he is innocent, he is going to vigorously defend the charges which are against him.

"They are very serious charges. He's resigned from the Liberal Party, he's not a member of the Liberal Party he needs to make that decision [whether to resign from Parliament] himself.

"I support the presumption of innocence until proven guilty."