Transmission Gully, the long-awaited $1.25 billion motorway out of Wellington, is finally expected to open this week.

The four-lane road has been bogged down with delays, construction defects and budget blowouts.

After a "huge amount of work" on the project over the summer, Waka Kotahi NZ Transport Agency has said it's now confident the road has reached a stage where it is safe for public use.

It has instructed the contractor to open the road to traffic by the end of March, which means by this Thursday at the latest.

Traffic travelling in different directions will be fully separated by median barriers, there are interchanges rather than intersections, and no walking or cycling is allowed.

The new motorway will take people through parts of the region they might not be familiar with, so Waka Kotahi has advised motorists to keep their eyes on the road.

"We know it may be tempting to sight-see along the route, but as with all motorways, there is no stopping anywhere on the road, except in an emergency."

Parts of the new motorway are located high in the hills and can be affected by strong winds, fog, and even snow or ice in winter. People need to remember to drive to the conditions and adjust their speed accordingly.

Waka Kotahi has advised there may be temporary speed limits in place when the road opens.

This is because one-third of the new motorway has an asphalt surface, while the remaining two-thirds is chipseal.

The chipseal surface needs to settle and driving too fast may send loose chips flying, or driving too slow may lead to chips sticking to vehicles.

Waka Kotahi has also told motorists to plan their journeys and familiarise themselves with the changes to their trips when Transmission Gully opens.

For example, the journey into Wellington from Porirua's city centre will be different to the route home.

In other locations, the new motorway might not actually be the most efficient route to take.

People driving to or from Mana, Plimmerton or Pukerua Bay should continue to use the State Highway 59 coastal route.

When the road opens, Transmission Gully will become State Highway 1 and will be the main route in and out of Wellington.

The road is designed to be safer and more reliable for motorists. It will also be better able to resist and recover from earthquakes and storms.

It's considered to be one of the most significant pieces of new road construction in New Zealand.