WA Labor have retained the Seat of Rockingham after a quick by-election decided on Saturday, despite losing a chunk of support previously supplied to Mark McGowan.

Twenty eight-year-old Magenta Marshall will be the new face of Labor in a stronghold for the party, but doubts are now building ahead of the state election next year.

Press PLAY to hear more about what happens post-Rockingham by-election

University of Notre Dame Professor of Politics Doctor Martin Drum told Gary Adshead on 6PR Mornings each party was already attempting to spin the results to their favour.

“Labor retains the seat with a comfortable margin, but they lost 30 per cent of their primary so the Libs are saying they’ve won ground,” Dr Drum said.