Contentious new research concludes that cycle helmets do not protect riders from injuries as much as previously thought, but a New Zealand expert urges cyclists to keep their lids on.

Various reviews have found wearing a helmet - compulsory in New Zealand since 1994 - reduces by at least 60 per cent the risk of head injury in a crash.

But now political scientist Dr Rune Elvik, of Norway's Institute of Transport Economics, has recalculated the head injury risk reduction at 43 per cent.

Further, he argues, recent studies show that when head, face and neck injuries are counted together, there is "no net protective effect" from wearing a helmet, because they actually increase the risk of neck injuries.

His review, published in Accident Analysis and Prevention, addresses what he considers flaws in earlier reviews - including one by the prestigious worldwide Cochrane Collaboration - such as their rejection of some weaker studies.

Professor Alistair Woodward, head of the School of Population Health at the University of Auckland, said, "Cochrane is usually regarded as the gold standard in pooling studies and deriving a conclusion."

He said the Cochrane and Elvik reviews both found a reduction in serious head injuries of around half.

"It's reasonably clear to my mind that helmets do protect people's heads and on balance they do more good than harm," said Professor Woodward, a helmet-wearing cyclist.

He said helmets were not designed to prevent neck injuries.

"Whether they cause the neck to bend more than otherwise, I suppose it's possible. If there is an effect [on neck injuries], it's much smaller than the protective effect from head injuries."

He accepts Dr Elvik's conclusion that modern soft-shell helmets have become more common and offer less protection than the older style hard-shell helmets.

"The first helmets were [made for] rock-climbing. Only later, people realised the energy-absorbing material inside the shell is probably what's more important ... and the surface of the helmet has become more vented and less rigid."

Professor Woodward said some argued that compulsory helmet-wearing deterred some people from cycling.

"My own view is that when we brought helmets back in the 70s and 80s they were pretty expensive and clunky. I doubt they are as much of a deterrent now as they were at that time."

Ministry of Transport national surveys going back to 2003 indicate around 92 per cent of cyclists wear a helmet. Not wearing one risks a $55 fine. Aucklanders' helmet-wearing dipped to 76 per cent in 2003 and was 88 per cent in 2009.

Police figures show 9618 tickets were issued last year for not wearing a helmet. The annual tally has generally trended upwards since 2000, when 5550 tickets were issued.

Cycle Action Auckland spokeswoman Barbara Cuthbert said helmets were a useful safety device but contentious for some cyclists who thought they portrayed cycling as dangerous, when most road crashes involving cyclists were caused by motorists who didn't look or didn't see the cyclist.

* New research indicates wearing a helmet reduces the risk of head injury in a crash by 43 per cent.

* Previous research found the risk reduction was at least 60 per cent.

* The new findings are disputed.