Forcing entry onto three vessels illegally fishing in the Southern Ocean would be too dangerous, Foreign Minister Murray McCully says.
A major naval operation has been underway against the ships, which are suspected of fishing illegally for toothfish in Antarctic waters.
Foreign Minister Murray McCully said crew from patrol vessel HMNZS Wellington have tried to board two of the vessels but both captains refused to allow them aboard.
Mr McCullly said overnight Equatorial Guinea agreed the three vessels - located in the Southern Ocean - were operating illegally and agreed New Zealand should board the vessels to verify their flagged status.
He said the Wellington attempted to exercise its legitimate right to board the Yongding and the Songhua earlier today but the vessels' captains refused to cooperate.
The minister said, due to the conditions and the evasive tactics of the ships' captains, it was not possible to safely board them.
He said it was disappointing that the navy has been unable to board but it was important the navy was kept safe.
"The judgment of the captain was that, in the conditions, and with the evasive action being taken, it would have been dangerous for them to try and forcefully board.
"So they've simply gained as much evidence as they can and called it a day, if you like."
He said New Zealand would look to share evidence of the ships' fishing activities with international partners.
'Difficult and dangerous'
Mr McCully said in an earlier interview that Equatorial Guinea, under whose flag the ships are operating, had granted permission to board the boats.
He said the commander of the Wellington was informed this morning the right to board had been granted.
"It's a matter for the judgement of the officers in charge of the vessel as to whether it's actually safe to do so. It's a difficult and dangerous part of the ocean. We don't know what evasive action the vessels might now take."
The navy ship has intercepted two foreign fishing boats in the area - the Kunlun and the Songhua. They were thought to be taking toothfish.
It has also caught a third ship, the Yongding, to the west of the Ross Sea, which it believed was part of the same fishing syndicate.
Mr McCully told Summer Report Equatorial Guinea would take steps to rescind the flag of any ship involved in illegal fishing.
Government officials were examining carefully structured commercial deals for evidence of the ships' ownership.
Mr McCully said there had been links in the past to a prominent Spanish sydicate operating vessels in the region.
Regulations on harvesting toothfish are managed by international organisation CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources).
The commision was established by international convention in 1982 with the objective of conserving Antarctic marine life and its 25 member nations agree to strict rules including catch limits and reporting fishing data,
Equatorial Guinea is not a member of the commission, and uses gillnets up to 25 kilometres long in length to fish, which have been banned under CCAMLR since 2006.
The commission's stance is that any non-member state fishing in the area is doing so illegally.