A fire the size of 53 rugby fields in Marlborough is putting up a fight.

Thirty firefighters are working to bring the blaze under control, after it almost doubled in size from 30 hectares yesterday afternoon.

Principal Rural Fire Officer Richard McNamara says the fire has a lot of internal heat, meaning it's going to take more than cold water to extinguish it completely.

"We have to physically dig this fire out and kill it just with hand tools, shovels and rake hoes, and follow that up with back packs and water sprayers and backed up by heavy tankers."

Richard McNamara says the fire won't be out before the day's end.