A New Zealand frigate involved in an international operation in the Western Indian Ocean has seized almost 260kg of high-grade heroin worth about $235 million.

The HMNZS Te Kaha has been working in a United States-led operation and seized the huge quantity of drugs after searching two boats.

Because the frigate was operating outside of New Zealand waters, the boats' crews were allowed to sail on after the seizures.

Small quantities were kept for testing, but the rest of the heroin was dumped into the ocean, using a fire hose that ran through a modified green wheelie bin.

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HMNZS Te Kaha participating in Operation Haku. Onboard suspect Dhow. Photo / Supplied
HMNZS Te Kaha participating in Operation Haku. Onboard suspect Dhow. Photo / Supplied

The drugs were likely destined for Africa and then onwards to Europe and other markets.

It is not known who or what organisations were behind the shipments, but the NZ Defence Force said such drug-trafficking was known to fund terrorist organisations.

NZDF Maritime Component Commander Commodore John Campbell said at a media conference this afternoon that the haul was a great outcome.

The involvement of New Zealand came after approval from Cabinet.

Te Kaha participating in Operation Haku. Photo / Supplied
Te Kaha participating in Operation Haku. Photo / Supplied

"Under the United Nations Convention of the Law of the Sea, there are articles in there that allow us...to be able to board vessels on the high sea if we have reasonable reason to think they are involved in illegal activities."

He said disposing of the drugs had been a trial-and-error process.

"We learnt this from the Australian Navy - by using just a big green wheelie bin, and then you can spray the fire hose into it...it means you don't get any blowback on the ship and it's all washed over the side."

The narcotics removed in Operation Haku. Photo / Supplied
The narcotics removed in Operation Haku. Photo / Supplied

Te Kaha was due to arrive home on August 2.

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Media were today shown video of Te Kaha's crew boarding a vessel and discovering drugs packed below deck, and later disposing of the drugs while wearing white boiler suits and masks.

On June 8 a boarding party from Te Kaha found 139 one-kilogram bags of heroin in hidden compartments on a medium-sized dhow that had been monitored overnight.

Preparing narcotics for disposal. Photo / Supplied
Preparing narcotics for disposal. Photo / Supplied

Five days later another dhow was boarded after being spotted by a Royal New Zealand Air Force P-3K2 Orion. A seven-hour search found 118kg of heroin.

Te Kaha has been working with the Combined Maritime Forces (CMF), a 30-nation naval partnership led by the US.

It aims to promote maritime security and defeat terrorism, prevent piracy and the illegal trafficking of people and drugs.