The party's WA branch confirmed Mr Small would fill the vacancy at a meeting on Saturday.

Mr Small will be in third place on the Senate ticket behind senators Michaelia Cash in the top spot and Dean Smith in second.

Dr Sherry Sufi will be fourth place.

The first and second candidates on the ticket are virtually guaranteed to secure a Senate seat, while the third is possible and the fourth and fifth placeholders extremely unlikely under the upper house's voting system.

Ms Cash was widely expected to take the top spot on the WA Senate ticket however there had been some conjecture about Mr Smith facing a battle for second place.

The WA Liberal Party will now write to Premier Mark McGowan and Governor Kim Beazley on Saturday evening to formally notify them of their Senate ticket recommendations.

There will be a joint sitting of parliament to endorse the recommendations.

Mr Cormann is Australia's nominee for secretary-general of the OECD.

The WA Greens have already chosen their lead candidate for the Senate, former police officer Dorinda Cox, replacing the retiring Rachel Siewert.

Labor is expected to wait until after the WA state election in March to pick their ticket.

Australian Associated Press