Victorians deserve a leader they elected, not a birthday gift to a Liberal stalwart after undoing the Baillieu premiership, the opposition says.
Opposition Leader Daniel Andrews said Victorians should not be fooled that this would be a fresh start, as Dr Napthine was sitting at the cabinet table while the coalition cut funding to education and health.
He said Victorians would be shocked and dismayed Dr Napthine is premier.
"This is a birthday gift to the birthday boy. It is not an honour. It is not an entitlement won through an election," Mr Andrews told reporters on Wednesday night.
"It is always preferable for the community to choose the leader of this state."
Mr Andrews said Mr Baillieu was the victim of his colleagues' disloyalty.
Mr Baillieu's leadership demise came after MP Geoff Shaw quit the Liberal Party, leaving the government to rely on his vote as an independent to remain in power.
Mr Andrews said he had no doubt Dr Napthine would try to convince Mr Shaw to return to the Liberal fold and shore-up the government.
"One might suggest that the Liberal Party have effectively franchised out to Geoff Shaw the choice about who leads the government," Mr Andrews said.
"That's very strange and very odd to think that a colourful character like Mr Shaw might well have been the person who chose the premier of this state rather than the people of this state."