The White Swan Hotel in Greytown was briefly evacuated on Friday night after an employee noticed a burning smell coming from somewhere inside the building.

Greytown fire chief Steve Meyrick said a small number of guests were evacuated about 11.30pm, while fire crews from Masterton and Greytown used a thermal imaging camera to try to track down the source of the smell.

It was eventually traced to an electrical fault in the office, Mr Meyrick said.

"It was a UPS battery back-up for a computer that's decided to have a meltdown."

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Traffic on Main St was briefly reduced to one lane with stop/go signs while fire crews checked the building.