Someone has been broadcasting pig grunts and abuse over police radios on the lower North Island.

The broadcasts have been happening for months and include poems and even singing.

The mystery man behind this may have access to a police radio.

A portable radio went missing in Ohakune in 2016 and could be involved in the broadcasts.

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District operations manager Peter Thurston told Fairfax the police regularly audit equipment.

Someone, however, has access to the radio, with listeners quoted by Fairfax saying they have heard a man singing "Old McDonald had a farm", completed with oinks during the pig verse.

The listeners also said the oinking sometimes goes on for minutes.

Sometimes the man broadcasts rap music, with lyrics that include derogatory things about police officers.

It is not known whether the person is using a police radio or found a way to broadcast into the same channel.

According to police, interfering with police radio and police communications is a risk to public safety.

The offender could face up to a year in prison.