A derelict church at the country's southernmost nudist camp has been the target of a suspicious fire overnight.

LISTEN ABOVE: Wilma Kremer, Southern Naturally Nudist Club's secretary, spoke to Chris Lynch about the fire

The rural site called Southern Naturally has been open for more than 50 years on Aparima Rd near Otautau, about 40km west of Invercargill, and has 18 members.

The fire service was called after a passing milk tanker driver spotted the blaze at 9.45pm last night.

Southern Naturally spokeswoman Wilma Kremer said the old wooden church building that burned to the ground was disused and its windows removed.

"We had planned to get rid of it at some stage - but someone's done it for us."

Spokesman Andrew Norris said four crews spent two hours putting out the fire, but no one was inside.

He said an investigator will be on site today to determine how it started.