It started with an oink.

Upper Hutt man Andrew Holt, an avid listener of police scanners, was at home on Sunday when he first heard a series of pig noises coming across the channel for the lower North Island.

"He sounds as if he's partly drunk or something," said the bewildered Holt.

The mystery man, who has somehow found a way of broadcasting himself over the police scanner channel, has been doing it for months, according to Fairfax.

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But Holt told the Herald he has only heard the man over the past few days, first making pig oink sounds, then singing Old MacDonald Had a Farm.

His scanner is able to pick up communication from Palmerston North, but if he comes any closer to Wellington he can no longer hear it.

When he first heard the pig noises, he thought: "Who's being that stupid? What an idiot.

"Sooner or later you'll get caught out."

If police were to catch out the oinking man, he could face up to a year in prison for criminal nuisance.

A police spokesperson said police were aware of the issue, and any interference with a police radio "constitutes a risk to public safety".

The man has also been heard performing poems, playing rap music, and singing obscene lyrics about police officers' mothers, Fairfax reported.

Holt said he hadn't heard anyone respond to the man.

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"He sounds a bit drunk or a bit strange. He didn't sound like a normal sort of person."

He expected the man had a bone to pick with police, or had mental health issues.