Firefighters are battling a large blaze which has taken hold of a forestry site near Blenheim.

The fire, around 80m by 40m, started shortly after 4am on the banks of the Wairau River.

It was sparked on a forestry block, in a section where workers bring the felled logs, southern fire communications shift manager Brent Dunn said. He said there was a lot of logging "slash", or debris, in the area.

Firefighters also had to face difficult terrain to reach the blaze, which is on a 30 degree slope, he said.

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The Fire Service has sent eight fire engines and two helicopters to the scene in a bid to dampen the flames.

"There's no risk to property or life, it's just an awkward fire to get to," Mr Dunn said, describing it as a "very" difficult fire to tackle.

There was no indication yet what caused the fire, he said.