Victoria's ambulance service insists all its reports comply with mandated frameworks after being accused of "creative accounting" on staff satisfaction.
The Ambulance Employees Australia Victoria union has made the accusation amid ongoing concerns about increased waiting times for the service.
Last month, Melbourne woman Christina Lackmann died while waiting six hours for an ambulance.
AEAV Secretary Brett Adie says Ambulance Victoria executives have hidden problems within the service.
"There is a clear pattern of behaviour here of presenting to the public and the government a service that was coping, when in fact it was not," he said in a statement.
"Questions must now be asked - who signed off on this and who stands to gain from hiding the real story.
"Today we have an ambulance service in crisis and part of that is because for many years issues were not addressed."
The union pointed to the Ambulance Victoria 2018/19 annual report, saying data manipulation was used to inflate positive responses about staff satisfaction.
"Ambulance Victoria has been caught out using creative accounting to publicly report misleading figures on the concerns of paramedics," the union said.
It said the AV figure of an 83 per cent positive response for safety and culture, meeting the goal of 80 per cent, was actually 63 per cent.
The union also alleged results had been manipulated on eight key statements put to staff in the service's annual People Matter Survey, which was published publicly for the first time last year.
"Ambulance Victoria initially told the government and the Victorian public they were not aware of the cultural issues and then stated they were not aware of the full extent when paramedics and the data suggested otherwise," the union said.
"The data proves the issues were well known for many years and AV failed to act."
But Ambulance Victoria said its reports were conducted, interpreted and published in line with mandated frameworks from Victoria's Department of Health.
"All methodology on how results are calculated is publicly available," the service's executive director of people and culture Rebecca Hodges said in a statement.
"The results of these reports guide and influence meaningful change within our organisation."
Ms Hodges added that everyone deserves to come to work every day feeling safe, supported and protected.
"That is why we are not only enhancing our service each and every day for both our patients and staff, but being open and transparent when we do have room to improve," she said.