"We recognise that the diversion is a major inconvenience for travellers and the freight industry, and our contractors are working hard to get it safely reopened," said Regional Performance Manager Mark Owen.
"There's been a huge amount of rain, and it has caused the river to rise and wash away parts of the bank that the bridge relies on for support.
"Safety is our first priority, and we need to make sure the bridge is safe and secure before it reopens to traffic."
Other areas in the lower North Island have been affected by wet conditions and flooding.
The SH53 Waihenga Bridge in Martinborough has been closed, with diversions in place.
Surface flooding has closed SH3 between Inglewood and Egmont Village, with no diversion in place.
Surface flooding has closed the Calico Line on SH1 between Bulls and Hunterville, with a diversion is in place.
A slip has reduced SH3 between Whanganui and Bulls to one lane at the intersection of Viles Road.
Surface flooding has caused multiple slips with trees down on SH4 between Raetihi and Whanganui.
As wet conditions and surface flooding are widespread, the NZTA urge motorists to take great care while travelling.
"Slippery roads are much more dangerous, as they can turn minor errors into major problems," says Mr Owen.
"Heavy rain and wind can also result in debris on the road, so keep your eyes peeled for any obstructions."
NZME. eo