Thousands of passengers have been caught up in cancellations and delays in the last few weeks due to mechanical failures and breakdowns.
Consumer chief executive Jon Duffy said the watchdog had received numerous complaints from people affected by the disruptions.
Under the Consumer Guarantees Act, ferry operators were required to carry out services "with reasonable care and skill", he said.
"Given the frequency of sailing disruptions, caused by mechanical issues, we do not think the operators are using reasonable care and skill in providing their services."
The act also required ferry operators to provide a service which was "fit for purpose".
"If a scheduled sailing is cancelled or substantially delayed due to mechanical issues then, as far as we're concerned, the operator has failed to provide reasonable care and skill or a service that is fit for purpose."
When the law was breached in this way, consumers would usually be entitled to damages relating to that breach, he said.
"Once people start having to fork out for accommodation and alternative travel arrangements, because of a cancellation or delay due to mechanical issues, we think the disrupted passengers are legally entitled to claim those costs back from the operator."
It was not enough for Interislander and Bluebridge to only offer to refund tickets or reschedule affected passengers on alternative sailings, he said.
"The operators are pointing to their terms and conditions which state they are not liable to consumers for costs beyond ticket refunds.
"It appears both operators are attempting to contract out of the Consumer Guarantees Act.
"We consider this to be a breach of the Fair Trading Act.
"As a result of this misleading behaviour, disrupted passengers are being left out of pocket. In our view, they're legally entitled to claim these costs back."
RNZ
