A big fire at a cheese factory north of Blenheim early today closed State Highway 1 and the main railway line until about 7am.

Firefighters were alerted about 3am and arrived at the Fonterra plant to find the main cheese hall area well ablaze, Blenheim chief fire officer Gary Price told NZPA.

"We have managed to contain it to that particular building, but it was a very large fire and a very large loss. Obviously a lot of expensive plant was inside."

Firefighters would remain at the scene to continuing dampening down efforts.

SH1 and the railway line had been reopened to let traffic through.

Mr Price said he understood the plant had been shut down for maintenance, and as far as firefighters were aware no one had been in the plant at the time of the blaze.

It had been necessary to close down SH1 during the fire because water had been brought to the plant from a river on the other side of the road.