Two schools and a stadium in South Auckland have been fitted with an alarm which emits an annoying sound that only people under 25 can hear.

The Mosquito Anti-Vandal System - labelled discriminatory against young people by overseas human rights groups - has been installed as Papakura council attempts to stamp out graffiti tagging and other offending in the area.

The electronic alarm, imported from Britain by an Albany company, is set at about 17.4 kHz and can generally be heard only by people under the age of 25 as the ability to hear high frequencies deteriorates with age, a condition known as presbycusis.

The sound has been described as similar to someone dragging their fingernails over a chalkboard.

The Mosquito box measures about 10cm across. It can be mounted on a wall or roof, has a 30m to 40m range and can be set to operate on a sensor or a timer, by remote control or be continuously running.

Peter Goldsmith, the deputy mayor of Papakura and chairman of the Safer Papakura Trust, said the council bought three alarms, two of which had been installed in the stadium at Papakura's Massey Park and the other at Papakura High School. Takanini School is also testing one.

The system, which costs about $1450, was trialled in August in a service lane between the railway and O'Shannessey St shops which is tagged almost every day.

It worked on a timer, coming on after 6pm when shopkeepers and customers had left the area.

Mr Goldsmith said council workers painted over existing tags before the trial to test its effectiveness. The first tags did not arrive until two weeks later and were far less extensive than normal.

The area was only targeted twice more in two months and both times with only one small tag.

When the Herald tested the device yesterday, the 22-year-old reporter was unable to tolerate the noise but it did not bother a 42-year-old photographer who described it as a "light buzzing" sound which was hardly audible.

A co-director of retail theft control company SLS, which imports the product, Graham Zuill, who is 54, said when the alarm was activated he still "had a strange sense that something was going on" but it was not unbearable for him to listen to.

He said the product had been tested by various agencies including the National Health Service in Britain and passed safety standards.

The sound does not disturb animals.

The Mosquito has caused controversy in Britain, the United Statesand Canada with activists sayingthat it infringes the rights of young people.

The Children's Commissioner for England, Professor Sir Albert Aynsley-Green, has sought to ban it.

The alarm is imported by the same company that distributes a synthetic DNA solution that has been installed at banks to douse fleeing robbers in a long-lasting solution.

* An electronic alarm emits high-frequency sounds only people under 25 can hear.
* Sound likened to fingernails dragged across a chalkboard.
* Can be set at a lower frequency everyone can hear.
* Used by the Papakura District Council at Massey Park, Papakura High School and Takanini School.
* Costs about $1450.