The WA Court of Appeal has ordered a retrial for a former police officer found guilty of tipping off a member of a white supremacist group that his telephone was being bugged.

Robert David Critchley was sentenced to 18 months jail last year, after being convicted by a jury of a charge of attempting to pervert the course of justice.

In 2010, Mr Critchley was working in the police department's telephone interception unit when he was alleged to have made calls from a public phone to tip off the target of the police investigation.

In a unanimous decision today, the three appeal judges ruled that Mr Critchley's conviction should be quashed and a new trial ordered.

The appeal court found that some of the evidence presented at Mr Critchley's trial should not have been allowed.