The Tramways Union says it's been concerned about Wellington buses for some time.
The comments come after the police laid 11 charges against a New Zealand Bus subsidiary, Wellington City Transport Limited, which trades under the name GO Wellington.
Police say the charges, including seven of failing to take action to avert serious harm, are in relation to a series of incidents in the year to September 2013.
Union secretary Kevin O'Sullivan, says it has held concerns about the buses, including safety, for some time.
He says about 12 bus drivers were interviewed as part of the police investigation late last year.
The chief operating officer of NZ Bus, Rachel Drew, says the charges lack substance, and the company will strongly defend them.
The case is due before the District Court in Wellington on Wednesday.
The Wellington Regional Council says the police charges are of serious concern.
The regional council's public transport portfolio leader Paul Swain says that given the council has several contracts with NZ Bus, the charges by police are of concern.
Mr Swain says the council expects all contractors to be fully compliant with all the relevant road safety requirements.
He says the matter is between the police and NZ Bus, but the council will be watching the proceedings with great interest.