Premier Mark McGowan's resignation prompts WA Labor left faction ...

Members of WA Labor's dominant left faction are set to meet this morning to decide who it will put forward in the race to replace Mark McGowan as Western Australia's premier.

Key points:The left faction looks set to pick Roger Cook or Amber-Jade Sanderson Rita Saffioti, who is unaligned, is the other possible contender for premierMr McGowan's successor has big shoes to fill, ex-premier Geoff Gallop says

In the aftermath of Mr McGowan's shock resignation yesterday, Deputy Premier Roger Cook has so far been the only person to declare his hand by revealing he wants the top job.

Mr Cook belongs to the left faction, which will attempt to reach a unanimous agreement on a single preferred candidate to nominate.

It will most likely come down to a choice between Mr Cook and Health Minister Amber-Jade Sanderson, who also belongs to the left faction.

Roger Cook (third from right) was among those watching on as Mr McGowan announced his resignation yesterday.(ABC News: Keane Bourke)

The faction holds power over the "progressive Labor" or right faction, and those who are unaligned in the middle.

The other possible contender is transport minister and close ally and friend of Mr McGowan, Rita Saffioti, who confirmed yesterday she was sounding out her colleagues.

Rita Saffioti has confirmed she's sounding out colleagues about the top job.(ABC News: Keane Bourke)

Like Mr McGowan, Ms Saffioti is factionally unaligned, but will likely have some support from the right of the party.

Leadership ballot could take weeks

The meeting of the left in Perth is due to begin about 10am.

It is unknown how long the deliberations will take.

Rules introduced at a state level mean if two or more people nominate for the top job, a lengthy process would commence, requiring a vote of all the party's rank-and-file members.

Like Mr Cook, Amber-Jade Sanderson is a member of WA Labor's left faction.(ABC News: Keane Bourke)

The main thing the party would like to avoid is a drawn-out leadership ballot, something that could take up to five weeks.

The key to avoiding that is settling on a replacement quickly.

Parliament is due to sit in a fortnight and some MPs would prefer a consensus candidate to enable a smooth transition from Mr McGowan's leadership.

Challenge for new premier: Gallop

Former WA premier Geoff Gallop said Mr McGowan's successor had big shoes to fill, but was confident about the party's future prospects.

"There's a lot of potential leaders in there so I'm pretty confident what will come out at the other end of the process will [ensure] good government continuing for WA," he said.

"Factions do play a part in Labor politics, Mark McGowan of course overcame that problem.

"He was seen very much as a person who was above the factions, so that's the challenge.

"What's important is that the person that gets the job then goes beyond that to make sure that he or she is governing in the interest of all West Australians."