Victorian Greens leader Greg Barber has had his last day in State Parliament after an 11-year career, announcing his immediate resignation today.

Mr Barber, an Upper House MP, had already signalled that he would not contest next year's state poll.

There had been speculation Mr Barber could run for one of the inner-Melbourne seats in which the Greens are challenging Labor, but he has now ruled out a Lower House tilt.

"We have an awesome group of candidates lining up to contest seats that the Greens can win at the state election," he said.

"Balance of power in both houses is a real possibility, but I won't be contesting."

Mr Barber, who previously headed Yarra City Council as Australia's first Greens mayor, entered the Upper House at the 2006 election and became the leader during the last term of Parliament.

"I've been a very busy Green for more than 15 years," he said.

"I'm going to have a little rest, do some fishing and think about my future plans."

Positions up for grabs

Mr Barber will be replaced by Moreland's deputy mayor Samantha Ratnam, who recently successfully moved her council's controversial motion to scrap official Australia Day celebrations.

The seven-member Greens party room will now have to elect a new leader.

No candidates have emerged yet, with Colleen Hartland unlikely to run given she is also retiring at the next poll.

Mr Barber's retirement comes at an interesting time for the Greens, as the party goes head-to-head with Labor in the Northcote by-election, where it has a strong chance of adding to the two Lower House seats it already holds.

A win in Northcote would damage Labor morale and cause further angst, after the party performed well in the corresponding federal seat of Batman last year.

The Greens are also posing a threat to Labor in the neighbouring state seats of Richmond and Brunswick.

Labor is also losing the advantage of incumbency in Brunswick after high profile MP Jane Garrett announced she was quitting the seat and seeking a move to a safe Upper House spot.

Australian Council of Trade Unions president Ged Kearney is seeking Labor preselection to replace Ms Garrett.

The former minister's decision to move seats has been met with anger within Premier Daniel Andrews' Government, with senior members accusing Ms Garrett of being "gutless" and "running from a fight".

Ms Garrett has defended her plan and cited her recent health battles as a reason to change.