A high failure rate has led to police being armed with new Taser cartridges, with longer prongs that can better penetrate thicker clothing.
It is understood at least some police have been using the new Taser cartridges, which also have a greater range, since January this year.
Sources say the change was made because the original Taser prongs were failing to penetrate heavy clothing.
Police figures show the X26 Tasers have had a greater than 25% failure rate since they were introduced nationally in 2010.
The failure was put down to either the probes failing to penetrate clothing, or missing the target altogether.
Replying to questions asked in January about the changes and why they were made, national operations manager Superintendent Barry Taylor this month said he was withholding responses because "the information would be likely to prejudice the maintenance of the law, including the prevention, investigation and detection of offences, and the right to a fair trial".
Tasers had been discharged 63 times since they were introduced nationally in 2010. They were ineffective in 17 of those cases.
Eight times, the Tasers detached from the subject or their clothing.
In another eight cases, either one or both probes missed the subject.
The reason the Taser did not work in the other case was still being determined.
Despite the figures, Supt Taylor said police remained confident in the effectiveness of the Taser.
Police continued to monitor its use, while "utilising opportunities to enhance its operational efficacy ...", he said.
He also withheld answers to questions about whether any new Taser capabilities had any implications from a medical point of view, or if staff using them were being retrained.
The original police-issue X26 cartridges operate optimally over distances up to 5m.
The specifications of the new cartridge are unclear, but on its website the Tasers' manufacturer says it also makes cartridges for the X26 to suit warm or cold climates, including one - the XP cartridge - that is "ideal for colder climates" and "has a longer probe to penetrate thicker clothing".
Another cartridge also has the longer prongs and adds a 10.5m range for situations "where the extra distance is needed"
Asked on a recent visit to Dunedin if she was aware of the changes to Tasers, Police Minister Judith Collins said it was an operational matter for police.
However, she said she was happy with any operational change that made police safer in their work.
Police Association president Greg O'Connor said it was important police equipment was adapted so it was right for local conditions.
"The more effective it is, the safer it is for both the [target] and the officer."
Asked, then, if different Taser cartridges were used in different areas, and why Tasers in certain areas were not issued with the new cartridge in the first place, he said that was a matter for police.
Green Party MP Keith Locke, who has been vocal about people's rights in relation to Taser use, said one of his main concerns about longer prongs would be that they had the capacity to injure someone badly if they penetrated bare skin.
If they were more effective, more people would be tasered, which had implications for other possible injuries in the tasering process.
He would like to see police make public more information about Taser use, he said.
How Tasers work
• X26 Tasers work by delivering a disabling 50,000-volt shock from a hand-held unit, via insulated wires to two prongs.
• The Taser is fired and propels the prongs outwards. The prongs spread as they travel outwards.
• Once both prongs are fixed on the target - each has a small barb at one end so they can hook on to clothing or skin - the circuit is closed and the electrical current can flow.
• The Taser manufacturer's website says the electrical current can reportedly penetrate up to 5cm of clothing.
However, many comments on Taser feedback sites refer to problems with the Tasers penetrating thick clothing.
