Two temporary bridges have been opened on the alternate route between Picton and Christchurch to cope with the increase in traffic caused by the closure of earthquake-damaged State Highway 1.

The steel bridges run alongside the permanent one-way bridges, and a third will soon be open to traffic.

Transport Minister Simon Bridges says having traffic flowing in both directions will reduce delays at pinch points.

"It's vital we ensure that people can travel safely on the alternate route through Murchison and Lewis Pass," he said.

SH1 was closed by huge slips brought down by the November 14 Kaikoura earthquake.

Mr Bridges says work is continuing at pace and SH1 is expected to reopen within 12 months.