Firefighters say it could be days before a forest fire that burned throughout the weekend near Oamaru is out.

The fire, which destroyed about 6,000 trees over 20 hectares in Herbert Forest, reignited again on Monday morning.

Waitaki chief rural fire officer Eric Spittal says parts of the ground are still smouldering.

He says he is optimistic firefighters can keep the fire under control but it may take several more days to fully extinguish.

Fire believed to be suspicious

The National Rural Fire Authority and the Department of Conservation are to investigate the fire, saying they believe it was deliberately lit.

Eric Spittal says the fire is definitely suspicious and there were already leads for investigators to follow up.

He says it may be some time before the investigation reveals whether the fire was lit maliciously.