A spokesperson for Interislander said all affected passengers had been moved to other sailing times.
Bluebridge Cook Strait Ferries were not affected by cancellations but a post on its website warned “sailings during this period could be uncomfortable and longer than normal”. There could also be delays to services.
Passengers would be advised by text and email of any disruptions.
Six Cook Strait crossings have been cancelled, with a further seventh running as freight-only.
Bluebridge apologised for any possible disruptions during the holiday season.
“Sea conditions in the Cook Strait are expected to deteriorate from Sunday, January 12 until Tuesday, January 14,” it said.
“We are monitoring the forecast closely and will update this alert, and advise customers directly via text and email, if sailings are impacted during this time.
“Our top priority is ensuring everyone’s safety while minimising the impact on our customers.”
A post on the Interislander website also said safety was a priority and was the cause of any last-minute cancellations or delays.
“Cook Strait is notorious for being one of the most changeable stretches of water in the world,” the Interislander website said.
“It can change from calm or light swell to rough in a matter of hours.”
Interislander passengers affected or concerned with seasickness were urged to take precautions and sit towards the rear of the ship rather than the bow which experienced more movement.
Staff also offered other tips, such as taking seasickness pills at the start of the sailing, eating before boarding, and sipping on ginger ale.