Mr Barnett held the seat in Perth's salubrious western suburbs for over 27 years.

The 67-year-old officially retired last month, almost a year after Labor's landslide state election win saw him step down as party leader and retreat to the backbench.

Liberal candidate David Honey, an Alcoa manager and president of the Kwinana Industries Council, is expected to comfortably win the safe conservative seat on Saturday.

Dr Honey, who has a chemistry doctorate, is a former WA Liberal president and won support from the Cottesloe selection committee over five other people, including mining executive and lawyer Emma Roberts.

Ms Roberts, 40, had the backing of deputy leader Liza Harvey and other senior members of the party who believed a young, talented woman was suitable.

Having said the criteria for the candidate should be based on ability, youth and gender diversity, leader Mike Nahan had to admit 59-year-old Dr Honey only met one.

But he described Dr Honey last month as an "extremely polished and capable" person.

Labor previously announced it would not contest the by-election, conceding they could not win the seat, which has been held by the Liberal party since its creation in 1950.

Three independents and candidates from the Greens, Micro Business Party and Western Australia Party have thrown their hats into the ring.

Australian Associated Press