Victoria Police officers have falsified more than 250,000 roadside breath tests in the state over a five year period, an internal investigation has revealed.
The force said it was made aware of reports that officers were placing a finger over the straw entry hole of breath testing equipment, or blowing into the straw themselves last year before an investigation was launched.
After reviewing 1500 preliminary breath test (PBT) devices and more than 17.7 million tests conducted over five years it found more than 258,000 PBTs, or 1.5 percent of all tests, had been falsified.
Professional Standards Command assistant commissioner Russell Barrett said the findings were disappointing.
"This conduct will not be tolerated, any member found engaging in this practice from today has been put on notice they will be investigated," he said in a statement.
"We let ourselves down, we've let the community down. It stops now."
It's believed the fake tests were largely made by general duties and highway patrol officers, particularly in rural areas.
Mr Barrett said an external investigator would be appointed to look into the issue further.
"The investigation will examine how this behaviour was allowed to occur and what we can do to enhance and improve our operational practice into the future."
Mr Barrett said he believed officers may have falsified the tests to meet targets.
"The question we all asked was why? There could be a number of reasons but the main rationale I believe is to hide or highlight productivity. Whatever reason our workforce may come up with, it isn't acceptable," he said.
"As disappointing as this is, it should be noted that, at this stage in the investigation, there is no evidence to suggest fraud or any criminality has occurred.
"Similarly, there is nothing to suggest that any of this activity has impacted on any prosecutions."
Mr Barrett said more oversight of preliminary breath testing would be put in place.
He said all officers would also need to attend a "guidance" session.
Victoria Police said it was in contact with the Independent Broad-based Anti-corruption Commission (IBAC) about the issue.