Fire crews are continuing to fight a large blaze on Chatham Island which has grown in size.

Firefighters have been battling a more than 2000ha blaze on the island since yesterday.

Fire and Emergency NZ principal rural fire officer Craig Cottrill said containing and extinguishing the fire would be a challenging and long operation.

"The fire has grown in size over the last 24 hours, and we are currently assessing how big it is," he said.

Two helicopters arrived at the island today and were working on the fire. Cottrill said they had made good progress.

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"We expect the teams who arrived on the island today to be there for up to two weeks. It is likely we will send additional resources, and two crews are on stand-by in Auckland, and are ready to travel to the Chatham's when required," Cottrill said.

"Cooler temperatures and rain have eased conditions today, and we do not expect to evacuate any residents, even as a precaution, as was done last night."

The fire is burning on private land containing regenerating and established native vegetation. It was too early to assess the impact of the fire on native plants and animals in the area.

Department of Conservation operations manager for Chatham Island Dave Carlton said at this stage the fire has not burnt into any conservation land in the southern area of the island

"Although this remains a potential risk," he said.

"The fire does not pose a risk to threatened native species such as the Chatham Island taiko, and is well away from the J M Barker (Hapupu) National Historic Reserve where the Moriori tree carvings, rakau momori, are found."

Residents who were evacuated overnight returned to their homes earlier today as the wild fires continued to burn.

Ten more firefighters were due to leave Wellington for the island at 3pm today.

"Fortunately last night the weather changed and there's a bit of a southerly," Cottrill said earlier today.

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"It's cooler out there today and the fire hasn't been burning as rigorously today as it has been the last two days."

A charter aircraft was also due to arrive today with fire crew carrying fuel, equipment and supplies for a two-week deployment.

The team of firefighters heading out from Wellington were specialist rural firefighters from around the region, and have been told to prepare for a two-week deployment.

Cottrill said they would be heading into "rugged terrain" that is "hard to get into, hard to get around".

The isolation in the area will be one of the biggest challenges firefighters face, he said.

When fighting fires at home in the Wellington region "if we need additional assets we can push a button on our truck and they'll magically appear".

But out in the Chatham Islands, firefighters are warned to take everything they need with them "because if you need anything more, it's going to be a day away".

A Hercules aircraft is tentatively booked to leave from Auckland on Monday along with two standby fire crews, depending on the state of the fire and whether more resources are needed.

Every Air Chathams flight operating over the next few days will be carrying food, water, and other resources.

Some of the firefighters will be staying at a marae while others will stay at a hotel.

"Think back to last year, the Port Hills fire. We are well rehearsed for this type of thing."

All the firefighters heading over today are volunteers, but will be paid for their time.

More rain is expected overnight along with a northerly wind shift.

Residents from 19 households were allowed to return back to their homes early this morning, after safety concerns prompted evacuations last night.

The island's civil defence emergency management team posted on Facebook at 7am that it was safe for the households on Waitangi Wharf-Owhenga Rd who had been told to leave their homes last night, to return this morning due to favourable weather.

"Due to lower humidity and some steady rain overnight the evacuation has been lifted...Those people who evacuated you are safe to return home at this time," the post said.

Residents were warned that wind and temperature changes might force further evacuations.

An incident management team is on the island and assessing conditions to ascertain the best way to fight the fire which has mostly covered conservation land.

The Defence Force and Department of Conservation staff also been involved in dousing the blaze.

Yesterday the Defence Force conducted a fly-over to determine the scale of the blaze.