The Waikato Expressway is closed southbound between Mercer and Te Kauwhata while the road is cleared after multiple crashes.
The highway northbound is open to one lane only.
Visibility is poor because of fog and motorists are being warned there are delays.
A detour via SH2 Mangatawhiri, returning to SH1 via Te Kauwhata is in place.
NZTA said the detour would add at least 30 minutes to the travel time and advises motorists to delay their journey or allow extra time.
The southbound detour route is left to Koheroa Rd to Mangatawhiri, then right on SH2, right to Okaeria Rd to Waeranga Rd (via Te Kauwhata) left to return to SH1 southbound.