It will be a patchy Labour Weekend for much of the country, with motorists and trampers warned to take extra care.

The combination of capricious weather and the Rugby World Cup had road safety advocates pleading with Kiwis to take road safety seriously.

Road safety charity Brake said people should remember that drugs and alcohol could stay in a person's system for longer than might be imagined.

"With the Rugby World Cup reaching a crucial stage, and many people getting up early to watch the match, Brake is reminding drivers to make sure they're fit to drive before setting off, and during the journey," the charity said.

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"That means having enough sleep before a journey, taking regular breaks during a trip, and not drink or drug driving."

Drivers should take at least a 15 minute break from driving every two hours during long journeys, and get a good night's sleep before setting off, Brake said.

"Family and friends also have a part to play in saving lives, by speaking out to loved ones who are considering getting behind the wheel when they are tired, or after they've consumed alcohol or drugs."

Superintendent Steve Greally, national manager road policing, said the crash risk for fatal or serious crashes was 14 per cent higher nationally over Labour Weekend.

Police would issue "yellow cards" to dodgy drivers, Mr Greally said.

"The yellow card reminds people that if they incur 100 demerit points over a two year period, they will have their licence suspended for three months," he said.

"We'll also be carrying out random alcohol checkpoints over the weekend and will be breath-testing anyone stopped, anywhere, at any time."

A reduced 4km/h "speed threshold" would be enforced from Friday afternoon to Tuesday morning, police said.

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Craig Foss, associate transport minister, said he was worried about risks to people travelling on the roads this Labour Weekend.

"Sadly, this year's road toll is likely to be higher than last year's. It's important to remember each and every number in the road toll represents a lost life -- a parent, child, brother, sister or friend who will be greatly missed."

Mr Foss advised motorcyclists to check their bikes carefully before heading off.

Northland District Health Board chief medical officer Dr Mike Roberts said a spate of crashes over the past fortnight had left people with serious injuries, needing extensive surgery and months of rehabilitation.

"Of concern was one of these crashes had involved two children who had not been restrained in car seats resulting in both ending up with broken femurs."

There were three steps people should remember, Dr Roberts said.

"Slow down, wear seat belts, drive sober and take breaks."

Drivers must be patient, as there would be more traffic than normal on roads, he said.

Mountain Safety Council chief executive Mike Daisley said people heading into the wild, even experienced trampers, should plan their trips carefully.

"Although temperatures are rising, conditions are still wintery around much of New Zealand," Mr Daisley said,

"On the popular Tongariro Alpine Crossing and on many of the southern Great Walks - favourite locations for Labour Weekend getaways - there's often still snow cover at this time of year. Parts of these tracks may even be affected by avalanche conditions."

It would be a moist start to the weekend for many areas, MetService said.

A band of heavy rain would stall over the central and upper North Island from Friday through to Sunday morning. Conditions would probably brighten on Monday.

"South of the front the lower North Island will be cloudy with a few showers, while the South Island stays mostly dry in the east and showery in the west, until another front arrives late on Monday," the agency said.

MetService issued a Severe Weather Watch for heavy rain about the central North Island hill country from the Tararua Range to Tongariro and Taranaki.

"People heading into the hills should take care, as these prolonged and heavy falls of rain will lead to rises in river levels," said meteorologist Tom Adams.

Northland, Auckland, the Bay of Plenty and East Cape had a chance of some heavy falls on Sunday.

"As the low pulls away on Sunday afternoon the weather improves for the upper North Island, but the lingering cloud could still bring some patchy drizzle."

A series of weak fronts would sweep over the west and south of the South Island but these would "fizzle quickly to patchy cloud", MetService said .

Another front would slink up the South Island on Monday, shrivelling the mercury down South and brining rain to the North.

"The east coast of the South Island will get a few showers Sunday morning, but stays mostly fine until Monday night."

The official Labour Weekend holiday period begins at 4pm on Friday and ends at 6am on Tuesday. Last year the Labour Weekend road toll was 3, from 78 reported injury crashes.