The private investigator hired by Western Australia's Liberal state government to quiz an opposition MP over a political leak says he has also worked on behalf of the Labor party in the past.

WA's Labor leader Mark McGowan and his party colleague Chris Tallentire voiced their outrage over the weekend after approaches by the investigator who was hunting the perpetrator of the leak about former Minister Murray Cowper.

Days before the March 9 election, it emerged a departmental inquiry had been launched into an allegation Mr Cowper had illegally cleared land at a holiday shack community on Wedge Island, 30km north of Lancelin.

Mr McGowan said the government's response to hire an investigator at a cost of more than $5000 was to waste money on a "witch hunt" to track down the whistleblower.

But that investigator, Joe Baskwell of Australia Wide Investigations, said he was merely providing a similar service to a plumber hired to fix a toilet at Parliament House - albeit for $171 per hour.

"They used to have a maintenance man who fixed a leak in a toilet, and I was also looking for a leak," Mr Baskwell told ABC radio.

Gosnells MP Mr Tallentire said he had felt threatened by the approach, while Mr McGowan said he had never heard of a private investigator being used by a government.

However, Mr Baskwell said he had been used two years ago to look into allegations by a Labor MP that a staffer was leaking information to the Liberals.

"(I was not harassing Mr Tallentire) I just did a good job," said Mr Baskwell.

Government documents show four investigators from the same company have certification to allow them to investigate on behalf of government agencies.