The New South Wales Opposition says timber mills on the north coast are struggling financially because of a shortage of supplies.

Primary industries spokesman Duncan Gay says one mill at Tyndale has already let go six employees because of the shortfall in timber.

He says many mills have already taken a 20 per cent cut in what they receive, despite having entered into a 20 year wood supply agreement with the government.

"Our concern is that there just isn't enough resource there and they signed off on a quota that wasn't sustainable," he said.

"We were concerned at the time that the Greens and the Government were making false promises.

"If they're not able to supply this quota, it's an indication that we were right."

Primary Industries Minister Ian Macdonald says heavy rain in the region have made it difficult to harvest timber.

He says Forests New South Wales has had to look at alternative measures to deliver timber to the industry, which is part of its agreements with mills.

"There's been no breaking of contracts," he said.

"Under the contracts at points like this, when you have heavy rain, which makes it very difficult to access a lot of the harvest areas, there has to be an arrangement made to ensure that timber is supplied.

"And that's precisely the process that's being delivered at the moment."