Police are defending the effectiveness of Taser stun guns after they failed to subdue armed offenders in two separate incidents in the past week.

Police in Auckland last night shot a 38-year-old man after a Taser proved ineffective, while the stun gun also failed during an incident in Christchurch last week in which two officers were shot and a police dog was killed.

Superintendent John Rivers said today the number of times Tasers had failed was similar to overseas rates, and there was nothing to suggest there was an issue with their use by New Zealand police.

Tasers had been discharged 30 times since they were rolled out nationally, and in all but four cases their use had been successful.

They were also discharged 16 times during initial trials, two of which were unsuccessful.

"The Taser is viable, it is effective, and it is delivering positive results," Mr Rivers said.

"It is normal to expect that, on occasion, the Taser will not be successful and there are a range of factors which might make contribute to this, such as both probes not making contact, or getting tangled in clothes."

Staff were trained to be aware of the possibility of an unsuccessful discharge, he said.

The weapon had proved to be effective when deployed and in many cases the threat of its use was enough to subdue an offender, Mr Rivers said.

Police Association president Greg O'Connor said Tasers were not designed to be used against armed offenders.

"There are times when a firearm is absolutely the only option," he told NZPA.

Prime Minister John Key said today that the failures were "an operational matter for police".

"I don't think you can say the Taser doesn't work. I think what you can say is there are incidences where the Taser has failed."

Tasers could be ineffective if they were deployed against people who were "very high on drugs", he said.

"It's possible for someone to get through that - it's a very high pain threshold, but it's possible."

He reiterated his support for Police Commissioner Howard Broad's plans to arm more police.

"I think that the moves the commissioner is making are understandable ... and he'll have my support in doing that."

However, Mr Key said arming more police was not "a panacea" to the problem, and his first choice was for police to use non-lethal options.

A campaigner against the use of Tasers told One News that police need to be given more training before being sent into the field with the weapon.

Marie Dyhrberg from the Campaign Against the Taser group says the training should be brought into line with firearm education.

"Given that the taser itself is potentially lethal and is a form of firearm, the training should at least be the same," she said.

Currently, police go through an eight-hour course before they can use a Taser, compared to nine days of training for a firearm.

Mrs Dyhrberg said if police do not recieve an adequate level of training with the taser, then they will resort to using a live gun.

"You're going to have police who have a lack of confidence in [The taser] and they're not going to deploy it properly, therefore it may mean that the use of a firearm may be the decision that is made when it ought not be be," she told One News.

- NZPA and NZ Herald staff