Thirty firefighters, three helicopters, and a six tankers are attacking skid fire on a forestry block in Marlborough.

Fire crews were alerted to the blaze around 5.30 this morning, near the Wairau river, west of Renwick.

Principal Rural Fire Officer Richard McNamara says because the fires on a logging site, it's not easy to put out.

He says it's burning in very heavy fuel, so it's unlikely to be put out by the day's end.