The Transport Agency plans to take a tougher line managing Lewis Pass this winter, to safeguard drivers unused to snow and icy conditions.
The North Canterbury alpine pass on State Highway 7 is part of the alternate South Island highway, which has seen a four-fold increase in traffic diverted by the closure of State Highway 1 through Kaikoura.
The agency said that in the past, snow and ice meant the pass could remain open to restricted vehicles, such as those carrying chains, but similar conditions this year meant it would be closed to all vehicles.
The move is backed by the National Road Carriers Group, which said delays caused by crashes could be longer than those caused by the weather.
The transport agency's journey manager Lee Wright said they did not want to take any chances with the number of vehicles on the road this winter, and that drivers should always check road conditions before leaving on a journey.
"We really are trying so hard to keep that alternate route open for everyone - especially freight, locals, and travellers. Again, it's around driving with care and patience, maybe adding a bit of time to your journey and knowing a bit about what's happening on that route before you actually go," Ms Wright said.
Grant Turner of National Road Carriers said it would cause issues for freight, but closing the pass to all traffic made better sense than restricting vehicles in heavy weather.
"We have a situation that if we do have an accident on Lewis Pass - with the amount of traffic going through there it may well take longer to clear than to clear the snow and ice when there is a closure on the Lewis."
Mr Turner said the transport agency had assured the road freight industry it would manage any closure as best it could. He said the plan was presented to an industry consultation group in Christchurch, which discussed management of potential winter closures around the country.
"We were aware of it, but not everyone in the industry is in full agreement with it, and there was a feeling trucks could still go through but as we're aware all the trucks that used to go via State Highway 1 are now going through the Lewis Pass, which is now far more intensively used.
"Damage to the road could end up being catastrophic if we don't look after it," Mr Turner said.
Ms Wright said it was also a timely reminder to school holiday traffic to make sure there were plenty of provisions and warm clothing in vehicles, in case of delays.
The Transport Agency said there would be an extra police presence on the section of highway during winter.