Earthquakes and weather events are being blamed for continued delays to the completion of Transmission Gully

But, NZTA is expecting the highly anticipated 27 kilometre bypass north of Wellington to open by the end of next year.

NZTA acknowledges its original planned opening date of April 2020 will not be achieved, but expects it will be open later next year.

But the agency is refusing to confirm an exact opening date.

NZTA's general manager of transport services Brett Gliddon says the progress has been slower than anticipated due to numerous weather and seismic events.

That includes resources needing to be diverted to the 2016 Kaikoura earthquake.