It follows Chapman’s decision in August to reject a $40 million timber port off Kangaroo Island, where her family owns property and her late father served as the former MP.

The Planning Minister and Attorney-General at the time insisted she had no conflict of interest to declare in relation to the decision, which was taken despite her department recommending that the project – which had longstanding Major Development status – could be approved.

“I’m completely satisfied that there’s no conflict of interest,” she said at the time.

However, that question will now form the basis of a parliamentary inquiry after the Labor Opposition moved to establish a select committee to examine Chapman’s statements to parliament on the issue, and to consider “any conflict of interest or abuse of office by the deputy premier” in relation to her decision to reject the proposed port.

The committee will also examine “any failure to declare any pecuniary interest”, Labor frontbencher Tom Koutsantonis told parliament early this afternoon.

The motion was successful after crossbenchers, including newly-independent former Liberals Dan Cregan and Sam Duluk, voted to support Labor’s bid – which was passed 24 votes to 21.

It’s believed Liberal exile Fraser Ellis – who has not formally quit the party – abstained.

The crossbenchers voted with the Opposition to establish the committee.

The move is a dramatic show of strength by the independents to the Marshall Government months out from an election, with key legislation yet to be passed by parliament.

Leader of Government Business Dan van Holst Pellekaan told parliament the inquiry was a “mud-slinging expedition”, arguing there would be “no fairness under this committee” because “it’s pretty clear [Koutsantonis] has already made up his mind” about Chapman’s alleged conduct.

“We support the Attorney-General,” he said, saying the Opposition was trying to “throw a whole pile of mud and see what sticks”.

“It’s a completely inappropriate use of parliament’s time and resources,” he said.

“This issue has been canvassed in the media and in other ways and there’s absolutely nothing we’re aware to suggest this select committee should proceed.”

But Koutsantonis said the Opposition had received recent information “that confirms our suspicions that the Attorney-General has acted inappropriately on this matter”, after an earlier push for a privileges committee was rejected by Speaker John Teague.

He said the inquiry must probe “whether there is a deep conflict within [Chapman’s] decisions and indeed whether parliament may have been misled”.

If the committee does find any abuses of office or the ministerial code, it can “recommend any sanction”, he said.

“I am sure if the Attorney-General has evidence to exonerate herself she will provide that to the committee willingly and there will be no claims of executive privilege,” he said.

“Hopefully, this committee can… get to the bottom of this to make sure that the people of South Australia know that we are all equal before the law and that some people are not more equal than others.”

The independents also backed a move to allow the committee to begin sitting today.

It is expected to conclude next month.

In a statement, Chapman said: “This issue has already been dealt with by the Speaker and dismissed.”

“It is open to the Parliament to establish a Select Committee, as the State Government did into the Labor Party’s despicable publication of a racist poster at the 2014 election, which no one has claimed responsibility for,” she said.

“If this is the Labor Party’s Parliamentary priority, that is their prerogative.

“I have no further comment.”

The burgeoning crossbench could yet make its presence felt even further today, with ex-Labor independent Frances Bedford pushing to pass legislation to establish an independent Speaker – which could force Liberal incumbent Josh Teague to resign from the party or even the chair.

The motion appears to have the support of the entire crossbench.