Ending a difficult week for the Andrews government, in which further pressure was piled on the Premier over the entitlements scandal, Mr Herbert said he "no longer [had] 100 per cent to give" after 15 years in state politics.

No byelection will be required, with Labor able to replace Mr Herbert in the upper house through a casual vacancy.

Mark Gepp, a veteran union official and one of Ms Garrett's senior advisers when she was emergency services minister, was quickly put forward as a contender for the spot. He is understood to have the backing of the left-wing unions.

Mr Gepp worked closely with Ms Garrett, who opposed a workplace deal for union firefighters that was supported by Premier Daniel Andrews. The position ultimately cost Ms Garrett and Mr Gepp their jobs.

Some senior Left faction figures hope Mr Gepp will be elected unopposed, while others want a rank-and-file ballot and a strong field to come forward.

The retirement of Mr Herbert, the former higher education minister, comes four months after he quit the cabinet after revelations he had used his government car to chauffeur his pet dogs, Ted and Patch, around.

First elected in 2002 to the seat of Eltham, Mr Herbert took the unusual step of moving to the upper house at the last election.

He underwent an operation to treat a rare form of stomach cancer in 2013 and his health concerns, while not life threatening, were challenging and one of the reasons he decided to move away from the marginal seat of Eltham.

Mr Herbert said the decision to leave now, before of a six-week break from Parliament, would give Labor enough time to appoint his successor.

He also said the timing was right for him to leave, with the release of an audit report into allowance claims for second residences showing he was not among those who abused the entitlements.

There had been media reports accusing him of inappropriately claiming the allowance, which Parliament said was incorrect.

"The report was out; there was no question about why I'm going," Mr Herbert said.

Mr Herbert is credited by some as rebuilding the Tafe sector after much funding was stripped by the previous government. A former teacher, he became close friends with Labor senator and powerbroker Kim Carr while working at Glenroy Tech 40 years ago.

"One of the finest people I have ever met and one of the finest Labor politicians I've known," Senator Carr said of Mr Herbert.

Mr Andrews said Mr Herbert's "work has ensured thousands of young Victorians will get the life opportunities they deserve with access to the training they need for the jobs they want".

The leader of the Liberal Party in the upper house, Mary Wooldridge, congratulated Mr Herbert on his career, but could not resist a light-hearted comment, saying Ted and Patch would be excited to spend more time with him.

With Josh Gordon