Vic Libs win by-election, another likely

One Victorian Liberal seat is safe, while another looks likely to stay with the party as counting continues in two by-elections.

The Liberals comfortably retained the south-western seat of Polwarth, with candidate Richard Riordan leading the primary first preference votes on 49.57 per cent.

In former premier Denis Napthine's seat of South-West Coast, Liberal Roma Britnell is leading the primary vote on 40.07 per cent.

Opposition Leader Matthew Guy claimed victory on Saturday evening in Polwarth, which had been held by former roads minister Terry Mulder.

"It's pretty clear by the polling that Richard Riordan has won, and he has won handsomely," Mr Guy told supporters gathered in Warrnambool.

Ms Britnell's biggest challenge comes from former Labor candidate-turned-independent Roy Reekie, who has 16.69 per cent of the vote.

Mr Guy acknowledged Dr Napthine's popularity with the electorate was a hard act to follow for a new Liberal candidate.

"From Denis Napthine leaving as one of the most popular local members in Australia, it is always going to be difficult," he said.

"I am very confident that from the feedback we are receiving ... we can get over the line."

South-West Coast has had 88 per cent of the vote counted, while 79 per cent has been counted in Polwarth.

Labor chose not to run candidates in the by-elections, ensuring the major challenge came from Nationals and conservative independents.

Counting will start again on Monday morning.