The net is closing on an anonymous writer with a "poison pen" sending letters to residents of a small South Island town.

Senior Constable Helen Fincham-Putter said three anonymous letters about Middlemarch residents have been written with a "poison pen".

"We have a right little busybody in town."

Two letters were sent to two separate wives and gave "unwelcome advice" on the "state of their marriages".


"If you think someone needs to know that information, you should be able to tell them face-to-face, otherwise it's just gossip and you should just keep your beak out," Fincham-Putter said.

The other letter sent to Mosgiel Police Station listed alleged disqualified and unlicensed drivers living in Middlemarch, a town of 300 about 80km northwest of Dunedin.

The letters were posted "out of town" but Fincham-Putter was sure the writer was living "right within" Middlemarch.

"They know stuff, more than anyone from the outside looking in - that's for sure."

A police forensics team said 90 per cent of the time the letter writer was a woman.

"Guys usually wouldn't sit down and go to that trouble to write a letter - the average guy would just say it."

Fincham-Putter had taken photos of the letters to try to match the handwriting with other handwriting in condolence books from funerals in Middlemarch.

The writer would be found, she said. "We may not know today or tomorrow but it will come out one day because someone will say something and it will all click."

If anyone got a similar letter they could talk to a trusted friend about it.

"Share it with someone you trust and objectively evaluate its contents."

Or the letter recipient could ignore it because if the writer "can't be bothered putting their name to it - I don't give it much credence," Fincham-Putter said.