KiwiRail is mothballing a Northland train line and locals fear the decision will force an extra 150 logging trucks onto the region's roads to cope with the added freight demands.

Following enquiries about a leaked email seen by the Herald, a KiwiRail spokesperson this afternoon admitted the contract with the only freight customer on the Otiria to Portland line expires at the end of August and is not going to be renewed. This will render the track useless but the line would remain open.

"KiwiRail is not closing its North Auckland line."

The leaked email from a KiwiRail manager said woodchip company Marusumi would instead build a roadway for its trucks.

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Transport minister Simon Bridges said the Government has no intention on shutting any lines but there was little or no demand on the line at the moment.

"In that sense, we can understand KiwiRail's perspective where they are seeking to run a commercial business."

Labour transport spokeswoman Sue Moroney said the not using the line would result in an extra 150 logging trucks on the roads making them more dangerous as more trucks meant more accidents.

And late last year, the Government announced a proposal to allow trucks and buses to be bigger and heavier despite an admission it could lead to more serious accidents.

Announced at the start of December, the changes to the Land Transport Rule: Vehicle Dimensions and Mass 2002 (the VDAM Rule) would allow trucks up to be up to 5cm wider and higher and one tonne heavier. The proposal received 220 public submissions.

"All those factors are a recipe for disaster. It's certainly not smart economics either."

A Portland mother said there was already a lot of trucks in the area as it is and if there was going to be more trucks on the roads, the routes would need to be upgraded.

The woman, who didn't want to be named, only lets her two young children play on their bikes on a Sunday -- the only day trucks don't run.

"You think putting that money into upgrading the rail, would far out weigh the cost of the expense of freight on the road, wouldn't it? You've got ACC payouts, counselling payouts on top of road maintenance. If you're going to put that many more trucks on the roads, then there's more accidents."

In 2014, 67 people died and a further 772 were injured in road crashes involving trucks -- 23 per cent of all deaths and 7 per cent of all reported injuries on our roads, according to a report by the Ministry of Transport.

It said because of their large mass, trucks tended to be over-represented in serious crashes. Deaths from crashes involving trucks make up around 18 per cent of the total road toll (5 year average), while just more than 6 per cent of the total distance travelled on New Zealand roads is travelled by trucks.

"This reflects the fact that, in a collision between a heavy vehicle and a light vehicle or vulnerable road user, there is a much higher probability of death or serious injury than in a collision involving only light vehicles."

Save Our Rail Northland organiser Alan Preston said the line was "vital" and provided a second form of transport which could be electrified that meant it wouldn't depend on the country's access to oil and on its prices.

"Maintaining the rail network is a strategic measure rather than an economic gain. The role that rail can play is so much more efficient for transport and for freight."