One of Jane Garrett's former senior advisers is a contender to replace Steve Herbert in the Victorian Parliament, after he sensationally quit politics.

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.