A large fire spreading through important wetlands in northern Waikato has burned through 477 hectares of scrub and bog and firefighters warn it will take another day to bring it under control.

The Whangamarino Wetland is a nationally significant site in the Department of Conservation's wetlands restoration programme.

DOC Waikato operations manager Niwha Jones said the fire was burning in the wetland's central bog - home to threatened bird species and one of New Zealand's largest carbon sinks.

Whangamarino was an internationally recognised wetland of significance covering almost 7000ha, with an "extensive mosaic" of swamps, fens, peatland bogs and open water around the Whangamarino and Maramarua rivers, he said.

It had been designated a Ramsar Convention on Wetlands site of international importance in recognition of its outstanding biodiversity, he added.

"The area where the fire is burning is known as the central bog and is the largest raised peat bog in Whangamarino - one of the few remaining in the southern hemisphere. This is a very rare habitat.

"Whangamarino is a known breeding site for threatened species like Australasian bittern/matuku-hūrepo, spotless crake/pūweto, and North Island fernbird/mātātā. It's also provides habitat for mudfish/waikaka and eel/tuna."

The wetland complex was also a hotspot for threatened plants, and was the only known site globally where the critically endangered swamp helmet orchid (Corybas carsei) could be found, he said.

"The bog is one of New Zealand's largest carbon sinks and this fire will be releasing some of the stored carbon back into the environment."

It was too early to determine the extent of the fire damage or how it would impact the ecosystem, Jones said.

In a statement, Fire and Emergency said two planes and eight helicopters with monsoon buckets were helping to fight the fast-spreading fire, with 40 firefighters on the ground.

FENZ incident commander Mark Tinworth said the fire had a perimeter of 10km.

"It's continuing to spread through the wetlands, but it's not threatening any residential or commercial properties at present," he said.

"There's a lot of smoke coming off the burning areas, and we advise people to keep car doors and windows shut if they have to drive near the area."

Island Block Road was closed to all except for emergency vehicles and residents.

Tinworth said FENZ was expecting it to take "another day or so" to bring the blaze under control.

"This is a really beautiful part of the country with considerable environmental value, and we're doing our best to prevent it from being destroyed."

FENZ was asking local businesses to ensure they had plans in place for removing any dangerous material.

Water for the fire trucks and helicopter buckets had initially been sourced from the local reticulated supply, but by Tuesday afternoon was being sourced from a pond on a local farm.

-RNZ.