Commuters from Waiheke Island got a stinky ride to work yesterday when a broken fan left smells from the sewerage system wafting through the cabin.

The Fullers Superflyte vessel had only just returned to service after two months of maintenance when the breakdown occurred.

Passenger Robin Tucker said about five minutes into the trip she and others started smelling "something like sewage" throughout the cabin.

It was so unpleasant she went outside, but the smell was out there too.

"All of a sudden there was this smell like air refresher. Someone said the staff had sprayed it but I'm not sure."

Ms Tucker said the experience reminded her of a "third world country" and worried what tourists would think.

"Imagine tourists getting on the boat to Waiheke and this being their experience."

Fullers chief executive Doug Hudson blamed the smell on the failure of a fan that was supposed to ventilate the sewerage system.

Sewage was not dumped in the harbour so the tanks were emptied onshore. The fans got rid of smells but when one stopped working passengers and staff noticed the difference.

Mr Hudson said the broken fan wasn't discovered when the annual maintenance was carried out. "I don't think too many passengers will be worried. They have their boat back, it has a new engine ... but it is a bit embarrassing."

A new fan had been ordered and would hopefully be fitted today.