Image copyright NZ Defence Force Image caption The New Zealand navy released what it said was photographic evidence of illegal fishing

The New Zealand navy is engaged in a stand-off with two boats thought to be illegally fishing toothfish in the Southern Ocean.

Naval patrol vessel HMNZS Wellington had been monitoring the vessels which are sailing under the flag of Equatorial Guinea, the foreign minister said.

Overnight authorities in Guinea gave New Zealand permission to board them.

But crew members barred authorities from boarding the vessels.

Foreign Minister Murray McCully said HMNZS Wellington tried to board the ships which was its "legitimate right", but the vessels refused to co-operate.

"Due to the conditions and the evasive tactics of the masters, it was not possible to safely board these vessels."

Image copyright NZ Defence Force Image caption HMNZS Wellington (left) has been monitoring the fishing vessels

Image copyright NZ Defence Force Image caption The area is protected and regulated - only certain countries are allowed to fish here

Image copyright NZ Defence Force Image caption The vessels appear to be targeting toothfish, also known as Chilean sea bass

While the ships are flying flags from Equatorial Guinea, Mr McCully said there was reason to believe they might be linked to a Spanish syndicate.

New Zealand was now working with Interpol to prevent the boats being offloaded at nearby ports and to investigate the links with the Spanish syndicate, the New Zealand Herald reported.

Mr McCully said New Zealand had strong evidence the vessels were illegally fishing toothfish, also known as Chilean sea bass.

"What we have already achieved is significant photographic evidence of what these guys have been up to down there," he said. "This is cynical international criminal activity and we need to stamp it out."

The Southern Ocean fishery is tightly regulated and cannot be fished by countries that do not belong to a multi-national conservation body.