A senior Northland firefighter is supervising ground crews battling a large fire on the Chatham Islands that is estimated to have burned about 3000 hectares so far.

Former deputy principal rural fire officer for Whangarei and Kaipara, Mike Sullivan, left on Saturday and has been appointed operations manager in charge of 10 firefighters.

He's expected back in Whangarei on Thursday.

The blaze, which broke out last Thursday, has ripped through land roughly equivalent to about 3000 rugby fields.

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Two helicopters with monsoon buckets have worked on the fire, supported by ground crews, many of them from Wellington.

The fire is expected to be fully contained in two days.

However, Fire and Emergency New Zealand said it could take two weeks before it is fully extinguished and any risk of it flaring up again is over, he said.

The fire is on private land containing regenerating and established native vegetation, but only about 1km from Department of Conservation land.

The cause of the fire is unknown, but fire investigators were expected to arrive on the scene yesterday.

The Chatham Islands are about 800km east of New Zealand and are home to about 600 people.