Gisborne police say they used the threat of a taser to subdue a man menacing mental health workers and police with two butcher's knives.
Human rights advocates say even though the weapon wasn't fired, it's disturbing that the mentally ill appear to be a major target of police tasers.
In the 15 months to the end of the March, tasers have been fired 15 times, including six times against people threatening self-harm and twice against dogs.
A spokesperson for the group Campaign Against Tasers, Auckland lawyer Marie Dyhrberg, says the fact that about half of those tasered by police have mental problems bears out her concerns.
"One of the most vulnerable groups who are going to be victims of being tasered are those who are mentally unwell, and unfortunately with all the physiological problems that are occurring with somebody in a highly agitated state, they are going to suffer a lot more."
The Director of the Centre for Mental Health Research at Auckland University, Brian McKenna, says analysis of data from the 2006-07 taser trial suggests about one in four taser incidents involves people with mental problems.
Although there is a potential for someone who is hyper-aroused to be vulnerable to the use of force, he says, the actual effects of tasering aren't yet well understood.
Professor McKenna says police are aware of the need to minimise use of force against the mentally ill and are working with mental health practitioners, human rights lawyers and community groups.