Up to 5000 trucks will be eligible to carry heavier loads on public highways from next month.

The change has raised concerns about motorists' safety from the Automobile Association and the Green Party, which are also worried about damage to roads from the juggernauts.

Transport Minister Steven Joyce has announced that trucks carrying up to 53 tonnes will be allowed on specific routes from May 1, subject to the new permit system.

Even heavier rigs would be allowed "in very specific instances".

The nine-tonne increase on the standard limit of 44 tonnes is the equivalent to the weight of nine small cars.

Green Party transport spokesman Gareth Hughes said trucks were already involved in 16 per cent of all road fatalities despite comprising only 4 per cent of the vehicle fleet.

"Bigger trucks have got more mass, more weight - that means worse accidents."

The Transport Agency believes 4000 to 5000 trucks could qualify for permits.

Although there will be no change to the height or widths of trucks, some will be allowed to exceed the standard 20m maximum length by 2m.

They include logging trucks, car transporters and vehicles carrying empty shipping containers.

Some buses will be allowed be 13.5m long - adding almost a metre - so bicycle racks can be fitted to them.

The Auckland-based Campaign for Better Transport says it has surveyed every local council in the North Island and found none with spare cash to strengthen bridges for heavier trucks.

Mr Hughes said Ministry of Transport figures showed the cost of bridge upgrades could reach $380 million.

KiwiRail expects to lose 15 per cent of its freight revenue to road carriers, although the ministry says other transport operators predict greater opportunities for transfers from rail to heavier trucks carrying bulk goods such as milk.

Ministry consultants believe the new truck rules could give productivity gains of 10 to 20 per cent, saving $250 million to $500 million a year, through fewer vehicles carrying a given amount of freight.

Mr Joyce said this would help to reduce road congestion, operating costs and vehicle emissions and improve road safety by slowing the increase in heavy vehicle movements.

Any vehicle issued with a permit for a heavier load would have to meet all appropriate safety requirements.

An initial proposal faced opposition from regional transport committees for most of the upper North Island, including Auckland.

Local Government NZ had highlighted concerns about ratepayers having to pay bigger maintenance bills and said councils would not issue permits for heavier trucks on their roads.

But the organisation's development and infrastructure manager, Geoff Swainson, said it was now "comfortable" with the plan.

"We're happier because we've got control over where trucks are going to go and there are now mechanisms to help mitigate the effects," he said.

AA spokesman Mike Noon said the Government had not given information about how extra road-user charges would be distributed to local roading authorities.

The ministry says local authorities will receive 50 per cent of the cost of extra wear and tear as Government subsidies, leaving ratepayers "who benefit from the regional stimulus and economic and community benefits" to meet the rest.

"We're happier because we've got control over where trucks are going to go and there are now mechanisms to help mitigate the effects," he said.

AA spokesman Mike Noon said the Government had not given information about how extra road-user charges would be distributed to local roading authorities.

The ministry says local authorities will receive 50 per cent of the cost of extra wear and tear as Government subsidies, leaving ratepayers "who benefit from the regional stimulus and economic and community benefits" to meet the rest.

On the road

* Trucks will be permitted to carry loads of up to 53 tonnes on specified routes.

* Some types of trucks, including logging rigs and vehicle carriers, will be allowed to extend to 22m "as of right" instead of by permits.

* Some buses will be allowed to be 13.5m long - up from 12.6m now.

* Farm machinery will be allowed on roads at all hours, as long as it occupies no more than one lane.