The Speaker of the Legislative Assembly issued a writ directing the WA Electoral Commissioner to proceed with a by-election following Mr Barnett's resignation last week.

Mr Barnett announced in December that he was quitting politics after 27 years in parliament.

He had stayed on in parliament after a landslide election loss last March that ended his Liberal-National government's eight years in power.

Nominations are now open and will close at midday on February 23.

Labor previously announced it would not contest the by-election, conceding they would not win the safe Liberal seat.

Candidates for the WA Liberals pre-selection include Emma Roberts, Deborah Brady, Andres Timmermanis, Sandra Brewer and David Honey.

Mr Barnett leaves behind a career as WA's second-longest serving post-war premier, including major infrastructure projects such as Elizabeth Quay and Optus Stadium.

The 67-year-old continued to comment publicly since his move to the back bench, criticising the new Labor government and his own government, most notably saying his second cabinet was weaker than the first.

The electoral roll for the by-election will close on February 14 at 6pm.

Electoral Commissioner David Kerslake said enrolment details could be checked and updated online or over the phone.

"Eligible residents in the district of Cottesloe need to make sure their enrolment is up to date, especially if they have recently changed address or turned 18," he said.

"You cannot vote in this by-election if your name is not registered for the Cottesloe district when the roll closes."