A backbench Liberal MP in Western Australia has launched a salvo against his own Government's use of speed cameras, saying they are becoming focused on revenue raising.

Nigel Hallett raised the issue in Parliament yesterday, saying he was unhappy that signs to notify drivers they had passed through a speed camera area were being used less frequently.

However, the Police Minister said that was partly due to occupational health and safety issues, with the decision on whether to use the sign left up to individual camera operator based on a risk assessment.

Minister Liza Harvey said in less than four years, there were 67 instances of an operator being threatened and four of them being physically assaulted.

But Mr Hallett, who represents the South West region in the Legislative Council, said that was "a cop out".

"If it was a real issue of safety they'd have two people in the car, the same as the police cars," he said.

"To me it's starting to point like more of a revenue raising exercise.

"It has lost the effect of the deterrent without the signs - I drive about 100,000 kilometres per year and I very rarely see a sign.

"In my opinion, and the majority of people who have contacted my office, it loses the whole point of combating speed and making people more away of their own speeds."

Ms Harvey said the issue was being regularly looked at but the advice of road safety experts was being followed.

"The evidence is very clear from New South Wales and Victoria - independent auditor-general reports have said speed cameras change driver behaviour, they have people slow down and they change lives," she said.

"Police advise me you need to have a combination of overt speed camera policing and covert speed camera policing and until I see evidence to the contrary that will always form part of our strategy."

Ms Harvey said the cameras served as a deterrent, regardless of whether they were visible or not.

The Minister foreshadowed changes to the signage and operation of speed cameras but said that would be guided by a consultant's report in the lead-up to the rollout of the next lot of cameras.