The number of helmet infringements was about 6000 a year in the early 2000s, but has sat above 8000 annually for most years since 2003.
Cycle use has increased too. In Auckland the number of daily cycle "movements" rose by 18 per cent in the five years to 2012, according to research done for local authorities.
The Ministry of Transport's cycle helmet use survey indicates helmet wearing was stable at about 90 per cent nationally in the decade to 2012, although it was typically slightly lower in Auckland.
Inspector Peter Baird, the national adviser on road policing policy and legislation, said people cycling without a helmet were putting themselves in danger.
"It's an important piece of safety equipment. It protects a vital part of the person's body in the event of a crash. It's too late taking it off your handlebars and putting it on when you're about to have a crash."
Alistair Woodward, professor of epidemiology and biostatistics at Auckland University, said, "My sense is that there are people who ride on the footpath, particularly in Auckland, maybe because they feel the roads are dangerous. A helmet might seem less important when riding on the footpath."
He said his research before helmet-wearing becoming compulsory in New Zealand in 1994 found that in crashes, cyclists without a helmet on their head suffered more-severe head injuries than those who wore one.
However, the Cycling Advocates' Network of NZ is calling for a review of the helmet law.
"It was expected to deliver a reduction in the rate of head injuries but it appears not to have done that," said the organisation's spokesman, Patrick Morgan.
Mandatory helmet-wearing laws could discourage some from cycling by making it seem more dangerous than it really was, whereas cycling had "massive health benefits and anything reducing the number of people cycling is a bad idea", Mr Morgan said.
"It appears that the more people cycling more often, the risk reduces for everyone, not just people on bikes. The crash rate goes down for pedestrians and cars as well."
