The navy may not be able to monitor illegal fishing in the Southern Ocean because its ships are not up to the task.
Foreign Affairs Minister Murray McCully said a navy patrol of the Antarctic fishery was cancelled because of concerns about its offshore patrol vessels' ability to operate in Antarctic waters.
Mr McCully said he was advised by the navy that the mission to tackle illegal or unreported fishing was not going to be possible because it was "not within the capabilities of the vessel".
Asked whether the navy needed better ships, Mr McCully said: "We were simply advised they weren't going to be able to achieve the mission and so they decided to pull it."
The HMNZS Otago had travelled to the Southern Ocean late last year to patrol the fishery with the support of the air force Orion, which has monitored the region since the late 1990s.
The 1900-tonne frigate's mission was described by the Government as a success, with serious fishing infringements uncovered by its crew.
Defence Minister Jonathan Coleman said the crew and the ship had worked in "the world's most extreme weather".
But a second patrol did not go ahead because of the navy's concerns.
The ship was one of seven vessels bought for $500 million as part of "Project Protector", and was designed to be able to stay at sea longer and carry out complex operations. The Government also bought a sister ship, the Wellington, which was capable of travel to the Antarctic.
The Otago was dogged by problems since its launch in 2006, and was not delivered to the navy until 2010.
The $90 million vessel was sent back after it was judged as not up to standard.
The Otago was later strengthened to withstand ice, but the increase in weight meant that it was unable to sail in Antarctic waters.
In 2010 it was found to have a leak which allowed seawater to get into its fuel tank.
Labour Party defence spokesman Phil Goff said he was "surprised" the navy was not able to carry out the patrol because the Otago's initial problems had been overcome before the vessel was sent to New Zealand.
The Defence Force was unable to confirm yesterday what shortcomings prevented the ships from making the second patrol.
The Otago and Wellington had strengthened hulls in case they encountered ice, but did not have icebreaking capabilities and could not enter ice packs.
It is also understood the Ministry of Foreign Affairs was concerned the navy could get involved in whaling clashes between Japanese vessels and the Sea Shepherd conservation group.
Whaling takes place in the same area in which the Otago was patrolling fisheries, and officials were concerned that the vessel could get drawn into the clashes, which have previously resulted in collisions.