Nearly half the people hit by Tasers had been threatening to harm themselves, police figures show.

However, the effect on these people is worrying mental health experts.

Auckland University's Centre for Mental Health Research director Brian McKenna said the Taser had a potentially traumatising effect on people with mental illness and could result in death.

"These statistics are of concern because you usually associate the use of force with threat to others, but this is indicating that the police are using force when there is a threat to a person themselves," McKenna said.

From December 2008 to May this year, Tasers had been presented by police 163 times, with 39 cases involving someone threatening to harm themselves or commit suicide, police figures show.

Tasers were used 13 times, with six incidents involving people threatening to hurt themselves.

Under Section 41 of the Crimes Act, police are justified in using force to prevent suicide or self-harm.

McKenna said the Taser was supposed to protect the public, but actually it was being used to protect people who were a danger to themselves.

"People can die in mental-health crisis when they get into a hyper-aroused state, and if the police use force, there could be a higher potential of adverse outcome or death," he said.

"Hyper-aroused" states could be triggered by drugs, alcohol, stress or mental illness.

McKenna said international research had shown links between hyper-aroused states, the use of force and deaths in police custody.

Police faced difficult decisions in assessing the mental state of someone threatening violence, but needed to be aware of the potential risks of using force.

"I'm not saying don't use force, that's a police decision," he said.

"They just need to manage the risks."

McKenna analysed data from a Taser trial period in 2006.

He has written an article that will be published in the International Journal of Law and Psychiatry in January.

A police report on the Taser trial included a March 20, 2006 case when a young woman who had locked herself in her bathroom and was displaying irrational behaviour was hit by a Taser.

An Amnesty International report found that the Taser was involved in 320 deaths internationally between 2001 and 2008, and most of these deaths occurred when people were in a hyper-aroused state.

Police Taser project manager Superintendent John Rivers said New Zealand police were "absolutely" aware that the use of force, including Tasers and pepper spray, against the mentally ill needed to be addressed.

"People experiencing mental illness are over-represented in our use-of-force statistics, and we are looking at alternative force methods," he said.