A Royal New Zealand Navy offshore patrol vessel is heading to the Pacific to help deter illegal fishing.

HMNZS Otago was set to sail from the Devonport Naval Base at 4pm to patrol the south-west Pacific for two months.

New Zealand Joint Forces commander Major General Tim Gall said vessels fishing for tuna would be the focus of random inspections.

He said the ship would carry New Zealand fisheries officers from the Ministry for Primary Industries as well as officers from Pacific Islands countries.