One of the candidates vying to be the next West Australian senator for the Liberal Party has described supporters of refugee-friendly policies as "a bunch of cockroaches" swarming all over his former electorate.

Michael Sutherland, the former Speaker in the WA parliament who lost his inner city seat in March, made the comments at a Liberal Party event in Sydney last month.

He is one of the five men revealed to be vying in this weekend's pre-selection for the Senate vacancy created by the retirement of Liberal Chris Back. Born in South Africa, Mr Sutherland has refused to provide proof he has relinquished his dual-citizenship, despite two Greens senators resigning in the past week after it emerged they were citizens of other countries.

Speaking at a panel discussion on how the Liberals can combat anti-fracking campaign groups, Mr Sutherland said such activists had overrun his former electorate of Mount Lawley.