Former minister Steve Herbert announced last Thursday he was quitting state politics after 15 years in Parliament, opening up a casual vacancy in the Northern Region. The announcement came four months after he quit cabinet following revelations he had used his government car to chauffeur his pet dogs.

Mark Gepp, a veteran unionist and former senior emergency services adviser to ex-minister Jane Garrett, was quickly put forward as a major contender for the seat.

But that plan has hit a snag as Labor's women's advocacy group, Emily's List, has said the spot must go to a woman under the party's affirmative action rules.

There had been a hope in some circles that Mr Gepp would be elected unopposed, while others were pushing hard for the rank and file to have a say in choosing Mr Herbert's replacement.

Mr Herbert's departure does not trigger a by-election.

According to party rules, 40 per cent of positions must be for women, especially in the case of a casual vacancy. Currently five of the 14 Labor MPs in the state upper house are women. Mr Herbert's retirement opens up the opportunity for the 40 per cent target to be met.

Public Transport Minister Jacinta Allan's chief of staff, Jess Howard, has been raised as a prospect and is understood to be considering a tilt.

Senior sources say other names are likely to emerge, but the preselection is fuelling tension within Labor's Socialist Left.

It is understood that the seat is claimed by a group centred on the CFMEU and other left-wing unions.

A lawyer by trade, Ms Howard was involved in a campaign against a poker machine development in Castlemaine before becoming a councillor in Mount Alexander Shire.

The prospect of a battle for the seat involving people linked to ministerial offices comes as figures reveal that the state government's budget for ministerial staff has blown out by nearly $3 million.

Figures provided to a parliamentary hearing show that in 2015-2016 the government spent $29.5 million on ministerial staff - the budget was $26.6 million.

The inquiry was told that as of June 2016 in the offices of the Premier and ministers there were 20 chiefs of staff, 10 ministerial directors, 187 ministerial staff and two casual staff.

Opposition spokesman Tim Smith said staff costs had risen by $3 million yet only 15 new full-time staff had been employed.

"That's on average $200,000 per new employee," Mr Smith said.

A government spokeswoman said Labor was getting on with the job of government.

"We've got the biggest infrastructure agenda in the country and make no apology for getting on with the projects that matter to Victorians - building the Metro Tunnel, new schools and roads, and removing our most dangerous level crossings," she said.