The Whirokino Trestle and Manawatū River Bridge, pictured under construction in 2018, will be in use in days.

Travellers on State Highway 1 between Foxton and Levin will soon drive over the new $70 million Whirokino Trestle and Manawatū River Bridge.

The NZ Transport Agency says the switch will happen in the "coming days".

Agency regional transport systems manager Ross l'Anson said although some finishing work needed to be done, the new 600-metre-long structures just south of Foxton were ready for use and would offer an improved route for freight and travellers.

"This section of SH1 is a vital link for the lower North Island and improving its resilience has been a key priority for the transport agency. The new structure is stronger, safer and looking fantastic.

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"The exact time of the switch isn't yet confirmed, but once it happens, motorists will simply be directed on to the new route as they continue their journey."

Traffic management and reduced temporary speed limits will remain while final construction takes place.

This could include stop-go controls, depending on construction activities and road surface conditions.

Construction of the "tie-ins", where the new and old roads meet at each end, was expected to take about eight weeks.

Southbound traffic wanting to use Whirokino or Matakarapa roads can follow the posted detour, which will loop them back over the old Manawatū River Bridge.

"While our crews are putting the final touches to this project, we're asking drivers to keep to the posted speed limits and take care around the site," l'Anson said.

The old Whirokino Trestle and Maawatū River Bridge were replaced because they didn't meet modern standards, the agency says.

The project to replace them began in 2017.