A fire started when a man tried to burn his toilet paper on the southern shoreline of Lake Pukaki.

A French tourist started a grass fire on the shores of one of New Zealand's most pristine lakes when he defecated and then tried to burn the evidence.

Twizel chief fire officer Simon Fox said the man, in his early 30s, had been walking the Te Araroa Trail along the southern foreshore of Lake Pukaki, Canterbury, about 6pm on Tuesday, when he found himself caught short.

The fire had started when the man tried to burn his toilet paper, Fox said.

"I was gobsmacked when he told me what had caused it, I had to give him a double take," Fox said. "It's the strangest fire call-out I have ever been to.

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"He was pretty embarrassed. He thought he was in the s***, so to speak."

The fire had quickly spread through the grass, reaching 25 metres by 10m at its peak.

Two fire engines from Twizel were called at 6.27pm and quickly put it out before helping with traffic control for the smoke, which was causing poor visibility on nearby State Highway 8.

The man was running around trying to put out his fire when they arrived, Fox said.

"To his credit I was suitably impressed that he hung around - most people in that sort of situation would probably have just kept going and not faced the music."

Fox said he explained to the man how defecation beside a lake was frowned upon by New Zealand residents and he suggested he invest in a poop pot from the Department of Conservation for the rest of his walk.

"In all seriousness we were quite lucky he did it on the lakefront side of the road because it would have had to jump the road to get any bigger," Fox said.

"If he'd done the same thing up in a forest in Blenheim we could have been looking at something much worse."