Police on the case were furious after they approached Buninyong candidate Michaela Settle, asking for an interview, in the wake of Ms Settle’s public comments on November 2.

Ms Settle, who worked as a field organiser for Labor in the 2014 election campaign, was reportedly interviewed by police in August when she was one of 17 Labor campaign workers taken into custody and questioned over the affair.

The candidate has always maintained that she had engaged in no wrongdoing over the use of parliamentary entitlements to pay for Labor election workers in the party's campaign that saw the first-term Liberal government of Dennis Napthine lose power.

Twenty-one Labor MPs, including six ministers, are involved in the scandal and none of them, on the advice of their lawyers, have spoken to police.

But Ms Settle, appearing with the Premier in Sebastopol on November 2, was asked by a journalist whether she was prepared to answer police questions.

"If I was asked to, I would always co-operate with police, of course,” the candidate replied.

But when detectives contacted Ms Settle in the days after the press conference, she exercised her legal right to not participate in an interview.

Ms Settle declined this week to discuss the matter with The Age and referred questions to the Labor campaign.

“As this is an ongoing matter it’s not appropriate to comment,” a campaign spokesperson said.

Victorian Police was similarly tight-lipped on Thursday.

“As the investigation is ongoing it would not be appropriate to comment,” a police spokesman said.

The Age understands that detectives on the Fraud and Extortion squad, already frustrated by the refusal of MPs to be interviewed, were left seething by Ms Settle’s backflip from her stated position on co-operating with police.

The interview process in the ‘red shirts’ investigation has already had several twists and turns with an initial plan by detectives to interview the MPs and ministers via questionnaire’s dropped in October after a political and media outcry.

The Coalition was infuriated later that month when The Age revealed that lawyers acting for some of the MPs caught up in the scandal had told police their clients would decline invitations to come in for questioning

The opposition, which is running hard on the issue as part of its election campaign, was continuing its pursuit of Labor over the scandal on Thursday with shadow attorney-general John Pesutto distributing private legal advice he has commissioned.

Mr Pesutto said the advice from barrister Gerard Nash, QC, stated that criminal offences may have been committed as part of Labor’s red shirts affair.

“He [Mr Nash] believes, on the evidence before him, that there were crimes such as obtaining property by deception, obtaining financial advantage by deception and possibly falsification by deception" Mr Pesutto said.

“These are all offences that are possible on the material before Mr Nash, that he has reviewed.”