A police radio, usually used for communicating police activity. The mystery broadcaster is believed to have been interrupting at random times of the day and night for several months.

A mystery man has found a way to broadcast pig grunts, obscene abuse, and rap music over police radio in the lower North Island.

The man has reportedly been broadcasting randomly for months, with anti-cop outbursts on police communications that include performing poems and singing.

On Monday an unidentified listener said the man was heard singing "Old McDonald had a Farm" over police radio – specifically the pig verse, complete with oinks.

The man has also sung an obscene verse of the same children's song, with lyrics about police officers' mothers.

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The oinking was commonplace, and sometimes ran in multiple bursts for minutes, the listener said. "I've heard him oinking, hard. That's mostly what he does."

Police said interfering with a police radio was a risk to public safety, and the offender could face up to a year in prison if caught.

The broadcasts are made randomly at all hours of the day and night, and occasionally "you can hear there are a few voices in the background, all laughing", another listener said.

"He oinks, and he plays rap music: 'F... the police, rah rah, rah', rappy-crappy stuff. And he'll actually talk over it ... derogatory stuff towards the cops. Sometimes he'll say little poems."

It appeared police were told not to respond to the man, but you could almost hear their "blood boiling" through the radio, the listener said.

On Monday, police said they were "aware of this issue".

They did not confirm whether the broadcasts were made using a police radio.

The broadcasts went out in the Central police district, which includes northern Kapiti Coast, Palmerston North, Norsewood and Feilding.

Police in the district still use an analogue radio system that can be listened to with a device called a radio scanner.

In 2009, police districts started changing to digital systems, which use an encrypted network that cannot be heard by criminals or the public.

Wellington was the first to make the change, and other digital districts include Auckland, Counties/Manukau and Christchurch.

Massey University senior lecturer Faraz Hasan said he had no specific knowledge of police systems, but it was possible to tune a transmitter to any particular channel and transmit.

"In this case, apparently, a channel that police use."

Digital transmission allowed "very robust security mechanism", generally not available in analogue communications, he said.