Waka Kotahi NZ Transport Agency has announced Transmission Gully, the troubled $1.25 billion road out of Wellington, is set to open to traffic by the end of the month. 

The four-lane motorway 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 says 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. 

Transmission Gully is being built through a public-private partnership (PPP), the Wellington Gateway Partnership (WGP), with CPB Contractors and HEB Construction subcontracted to carry out the design and construction. 

Under the agreement, the contractor WGP is responsible for determining when the road can be opened. 

Waka Kotahi will not be announcing a specific opening time to prevent any queuing, which could cause safety and congestion issues. 

The opening has been made possible by the agreement to defer some of the 100 quality assurance tests previously needed to be signed off before the road could open. Requirements for other tests have also been reduced. 

Waka Kotahi board chairman Sir Brian Roche said this was agreed during negotiations with the contracted parties. 

"Waka Kotahi has been committed to finding a pragmatic solution that ensures we are doing everything we can to open a safe motorway, while meeting the public expectations for it to be open for use as soon as possible and to ensure we safeguard good use of public money." 

However, wider negotiations over the road and previous delays to getting it open are continuing. 

Waka Kotahi transport services general manager Brett Gliddon said regardless of these negotiations continuing, the transport agency was confident the motorway was in a fit state to open. 

"We have not been prepared to compromise on the road's safety, however all of the critical safety assurance tests have now been completed to a standard that gives us confidence the road will be a safe, reliable route for motorists between Wellington and the rest of the North Island." 

Gliddon said now that quality assurance test requirements have been deferred, Waka Kotahi believed there was nothing substantial that could not be finished by the end of March preventing the road from opening. 

"We have told Wellington Gateway Partnership (WGP) that we expect the road to be opened as soon as possible and that responsibility now sits with them." 

A final pre-opening safety inspection will also need to be completed before the road can open. 

Gliddon said some pavement and road surface tests were among the quality assurance tests which have not been met. 

But he said expert advice was that the road is safe for use and there were benefits to allowing the road surface to bed in before winter. 

CPB HEB will continue to address any road surface issues over the next year as the chip seal continues to bed in, Gliddon said.