Long-serving WA Labor deputy Roger Cook will become the state's next premier after an extraordinary day of manoeuvring on Tuesday.

Mr Cook, 57, is set to become WA's 31st premier after spending almost 15 years as second-in-command.

Less than 24 hours after Premier Mark McGowan's shock retirement, West Australians were reminded how deep factional lines run in the Labor Party.

Mr Cook's cabinet colleague Amber-Jade Sanderson pulled out of the race late on Tuesday, a decision that came after the Australian Manufacturing Workers' Union delivered a fatal blow to her leadership aspirations, with Mr Cook securing the support of Labor's AMWU MPs in the afternoon.

Ms Sanderson vowed to respect the process and views of the entire WA Labor team after Mr Cook announced Transport Minister Rita Saffioti would be anointed his deputy.

"As I said this morning, my plan was to talk to my colleagues. I have now done that, and in the interest of unity and stability I will not be nominating for leadership of WA Labor," Ms Sanderson said in a statement.

"I look forward to working with Roger as the new leader of WA Labor."

Mr Cook's ascension to the state's top job is likely to be well received by First Nations leaders given his history working with Indigenous organisations.

In the early 2000s he became involved with Aboriginal advocacy groups, serving as a policy coordinator for the WA Aboriginal Native Title Working Group.

He was then CEO of the Yamatji Marlpa Barna Baba Maaja Aboriginal Corporation (now known as Yamatji Marlpa Aboriginal Corporation), and worked as a government relations manager for the South West Aboriginal Land and Sea Council.

Opposition Aboriginal Affairs spokesperson Neil Thomson said on Wednesday he hoped Cook's "affable disposition" would see him address the urgent issues facing First Nations people, particularly in the regions.

"There are many desperate needs in our Aboriginal community, particularly in our remote communities, and I can only hope that Roger Cook makes visiting remote and region WA a priority to hear first hand the challenges his government faces," Thomson said.

"I have spent the last two weeks visiting remote and regional communities and am shocked by the ongoing neglect.

"We have a crisis in housing and challenges with education and health services that need urgent attention."

Mr Thomson said he hoped Mr Cook would show more compassion and understanding of First Nations issues and address them appropriately.

"While Roger Cook has an affable disposition unlike his predecessor, who demonstrated snarly contempt when discussing the situation at Banksia Hill for example, Mr Cook's new Treasurer does not come with a good track record," he said.

"Rita Saffioti neglected to quickly address the transport crisis in the Kimberley, leaving many Aboriginal communities struggling to provide the basics such as green vegetables. I invite Roger Cook to go beyond Broome and hear first hand from Aboriginal people in the most remote parts of WA."