A witness has described seeing a woman's face catch fire this morning after a pie warmer  "blew up in her face".

The man, who wanted to remain anonymous, said he visited a food truck parked at Foodstuffs in Hickory Pl in Islington.

"I went out there this morning to get something to eat and I thought I could smell gas, but I just ignored it."

The pie warmer in the woman's food truck had gone out and, when she bent over to relight it with a match, it "blew up in her face".

"It happened pretty quickly. She patted at her face with her hands to put it out. It wasn't a raging fire but it was burning."

He said the woman took the incident "pretty well".

"She wanted to carry on working. She didn't scream or anything, but we made her sit down. Her face was pretty red, but it was hard to tell how bad the burns were."

The man said the incident was "surreal".

"It's just not something you expect to happen."

The woman, aged in her early 40s, was taken to Christchurch Hospital.

The incident happened about 8.45am.

The Department of Labour is investigating.

- © Fairfax NZ News