Forest and Bird is joining the crown's battle to establish the Kermadec Ocean Sanctuary.

It has announced its joining the crown in defending the sanctuary against Te Ohu Kaimoana, the Maori Fisheries Trust, which filed High Court proceedings against the Government in March.

The Trust says the sanctuary would clash with Maori fishing rights under the Crown's 1992 Sealord fisheries settlement.

But Forest and Bird's marine advocate Anton van Helden said the Government is well within its rights to establish the Kermadec Ocean Sanctuary.

He said they will make sure that fact's known in court.

"The Kermadecs is a really significant region. It's the only sub-tropical part of New Zealand's Exclusive Economic Zone and contains one of the most diverse deepwater habitats on Earth."

Mr van Helden said the sanctuary must be established, to send a really clear message that the world's ecosystems need protection.

"It's a very important statement for New Zealand, towards the rest of the world. It shows the significance of marine protection, and that we take it seriously."

The Kermadec Ocean Sanctuary, once in place, will close off 620-thousand square kilometres to fishing and mining.