However, a spokeswoman for NZ Bus said it wasn't an issue for most drivers.
"It's not acceptable," Mr Froggatt said.
"It's a bit embarrassing, it's not ideal and it's really stressful for the drivers having to continuously work for up to five and a half hours without a break."
Mr Froggatt said the union had advised bus drivers to stop and advise passengers they would be back in a minute, turn the engine off and go to the bathroom.
He said the only other alternative was to radio through to the bus company - NZ Bus - and ask for another driver to come out to the bus stop and take over the route.
"It's worse on the Link because you don't get a terminal break, we've said to the drivers, 'Look you're just going to have to [stop]'."It comes up at every meeting and the company keeps saying they're going to try and do something about it and everything, but it's just the way that Auckland Transport has structured it. Auckland Transport has a part to play in this process too."
One bus driver who wrote to the New Zealand Herald said he had been driving for NZ Bus for 10 years but the issue was worse with the Link bus routes.
He said there was a lack of public toilets located near the Link stops and NZ Bus did not allow drivers to go off-route or leave the bus while carrying passengers.
"Consequently, we often delay our need to use the toilet until we are off work," he said.
"These have caused us a lot of mental stress and are bad for our health."An NZ Bus spokeswoman said there was enough time between subsequent trips on the Link bus to allow drivers to take a toilet break.
"Passengers may remain on board the bus in-between trips, but we do not expect our drivers to explain the precise reason why they need to vacate the bus for a few minutes," she said.
A driver on a eight-hour shift wouldn't be operating the Link service for the full shift, she said, and all drivers take a break after five and a half hours.
"In general, our drivers manage their toilet breaks without issue.
"At the end of the day, if they have an urgent call of nature, then we expect that they will respond to this need as is only reasonable."
Auckland Transport bus services manager Brendon Main said the frequency of bus driver breaks was set by the bus operator.
The route and timing of buses on the Link circuit was set by Auckland Transport and did not provide time for breaks to make it efficient for commuters, he said.
He said it was about "getting the balance right" for drivers and passengers.
"We try to make the disruption as little as possible for the customers but the operators need to schedule relevant breaks for the drivers.
"How often that is we, as Auckland Transport, don't get involved with ... I know the locations where they do that and the fact that they do that, but whether it's enough is a matter between the drivers themselves and the bus operator."
Auckland Transport would have a conversation with NZ Bus about the situation to see if there was a problem, he said.
NZ Bus could also approach Auckland Transport with issues about routes at any time, he said.
