Under Covid-19 alert level 1, masks are mandatory on public transport, unless the person is prohibited from doing so for medical reasons.
Although it's not bus drivers' job to police it, Ministry of Health advice says they should encourage it where possible.
Bus and Coach Association president Alex Voutratzis said there has been a dramatic decrease in those wearing masks on board services, and it's putting drivers in an uncomfortable position.
"The driver's controlling a 15m-long piece of equipment, carrying 50-odd people who they're trying to get to their destination safely - they're not going to push them wearing a mask.
"It all comes down to trust because unless a driver is asking to see some kind of medical certificate, it's very hard for them to be challenging a passenger whether they should or shouldn't be wearing a mask."
Voutratzis said having to rely on a system of trust created serious challenges.
"You're always going to get people who try and take advantage of the rules and decide on that day that they may or may not wear the mask and push the boundaries."
Voutratzis said the guidance for drivers on the issue of masks was "murky" and there needed to be clearer definitions from the Ministry of Health around a process.
In a statement, a Government spokesperson said transport operators are issued with regular guidance including at every change in alert level.
"Operators, drivers and staff are not expected to assume the role of an enforcement officer, but they still have their usual customer relations role to remind people about face coverings."
Face masks or face coverings are one measure to help prevent the spread of Covid-19 and should be used alongside other basic hygiene measures like hand washing.
"You can go to the mall, cinema, walk through a town centre and be in a crowded place or event and not have to wear a mask, but the minute that you leave that situation and take a bus or a train you have to put the mask on.
"We firmly encourage passengers to keep wearing masks as it is our social responsibility and we cannot get complacent."
Lauren Bell travelled from Hastings to Wellington on an InterCity service earlier this month.
She said she was alarmed at the lack of mask wearing.
"The bus was sold out, and around three quarters of the bus were not wearing masks."
Bell said it appeared the driver couldn't have cared less.
"There was no mention from the bus driver throughout the entire journey that masks are compulsory on Intercity services, or in fact any mention of Covid at all."
However, general manager of New Zealand Coachlines Sam Peate disputed that.
"We have confirmed with the driver of this service that a passenger announcement to wear masks was made at major stops along the route."
He said some passengers don't play by the rules.
"Some customers do not adhere to the Government health order, but it's made clear that drivers should not act as the police in these situations."
