New Zealand's heavy artillery is off to the Pacific to guard against illegal fishing.

Navy vessel HMNZS Otago leaves from Devonport Naval Base this afternoon for a two-month stint in the southwest Pacific, where it will do fishing patrols and visit several ports.

Major General Tim Gall, the Commander Joint Forces New Zealand, said the vessel would help detect and deter illegal and unregulated fishing on behalf of several Pacific counties.

The shop will also dock in Fiji, Tuvalu, Samoa and Tonga "as part of defence diplomacy activities".

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"We have been supporting fisheries management in the southwest Pacific by deploying our ships and aircraft to conduct these annual patrols. Vessels fishing for tuna will be the focus of our inspections," Mr Gall said.

Fisheries officers from the Ministry of Primary Industries and several Pacific Island nations will be on board.

Meanwhile, an air force P-3K2 Orion aircraft will undertake surveillance in support of Otago's sea patrols.

"Overfishing of tuna stocks affects Pacific Islands countries," ministry compliance manager Gary Orr said.

"By conducting these patrols, we help progress the Pacific-wide campaign to educate and improve compliance with existing laws and regulations."