Motorists travelling the alternative route between Christchurch and Picton have been caught up in heavy congestion today, as traffic pressure mounts on the post-quake thoroughfare.

With landslides and quake damage closing SH1 around the Kaikoura area, the inland alternative route has been used by four times the usual amount of traffic, including more truck and trailer units.

Paul Mcdonald told Fairfax traffic had backed up as crews worked to repair a bridge between St Arnaud and Kawatiri, which had been closed for resealing.

"The road looks to have caved in because of the loads going over it," he said.

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"On our side, at a guess there are at least 50 cars waiting."

Only "half a dozen cars" had been allowed over the bridge in the time he was waiting, he told Fairfax.

The NZ Transport Agency is advising people using the inland route to allow at least seven and a half hours to get between Christchurch and Picton.

"Drivers need to be extra cautious and patient," the agency stated.

"When possible to do so safely, slower drivers should pull over to let traffic behind them through."

The alternate route was narrow in many places and speed restrictions were in place in some areas.

"Drivers should strictly adhere to the road signage and speed restrictions."

Police patrols would be more visible along the alternate state highway route and would be enforcing speed limits to ensure safety.

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Meanwhile, milk tankers and other essential freight would be given priority access in and out of Kaikoura on the inland road (route 70) out of town from next week.

Weather depending, the first trip by full milk tankers will take place tomorrow.

"While the inland road between Kaikoura and Mt Lyford is still a very fragile and hazardous route, crews have been working hard to clear the road of more than 50 slips to get it to a state where it can now be used safely, with vehicles travelling in supervised convoys," Transport Minister Simon Bridges said.