Claims officers were cheating the tests, by placing a finger over the straw entry hole or blowing into the straw themselves, were first reported late last year, Victoria Police said in a statement on Thursday.

"The investigation, which analysed over five years of data, 1500 preliminary breath test devices and more than 17.7 million tests, disappointingly found 258,463 PBTs or 1.5 per cent of all tests had been falsified," Assistant Commissioner Russell Barrett said.

"I had not heard of our members engaging in such a practice, we let ourselves down, we've let the community down. It stops now."

An external investigator will now be brought in to reveal why the duping happened, but Mr Barrett said the main rationale could be to "hide or highlight productivity".

The main culprits were general duties and highway patrol officers, with some rural areas over-represented.

"Whatever reason our workforce may come up with, it's unacceptable," he said.

There is no evidence at this stage to suggest fraud or any criminality had occurred, or prosecutions were risked, Mr Barrett said.

Statisticians had to look at complex algorithms together with considerations on the length of time it would take to administer one test and a succession of tests.

The external investigator will also look at underlying cultural and behavioural issues, supervision and management practices that let the fake tests go unchecked, and aggravating circumstances that may need further investigation.

Victoria Police is in discussions with the Independent Broad-based Anti-corruption Commission about the discovery.

Australian Associated Press