The main highway into Wellington is damaged after Cyclone Gita whipped up waves that crashed across the road.
Motorists are already facing heavy delays and southbound traffic into the capital is backed up as far as Raumati.
State Highway 1 was closed between Pukerua Bay and Paekakariki about 12.30am after it sustained weather damage overnight as the high tide washed across both lanes.
The storm surges caused significant damage to guardrails and parts of the footpath on the seaward side of the road collapsed.
Road crews worked throughout the night to clear the debris and the southbound lane heading into Wellington was re-opened shortly after 6.30am.
However motorists were warned to expect "significant delays" and the NZ Transport Agency is urging people to consider delaying trips if possible.
"The Paekakariki Hill Rd is available as a detour for light vehicles only, this detour can add up to 30 minutes to your travel time," police said.
Traffic backed up past the turn-off to Paekakariki Hill Rd after the closure is causing further delays.
Police hope to open the southbound lane as soon as possible but the northbound lane will have to be assessed in daylight.
South Island roads
Meanwhile two main West Coast routes are cut off this morning.
The road is closed between Jacksons and Kumara and between Haast to Makarora because of strong winds. A number of other main roads around the South Island are also closed after Cyclone Gita, and will be reviewed this morning.
In Canterbury, the Kaikoura Inland Route 70 is closed, plus most of State Highway 1's Kaikoura coast between Hundalee and Clarence. A slip has closed the road between Riwaka and Takaka.
• SH60 Riwaka to Takaka – Road closed (slip)
• SH6 Tahunanui to Stepneyville (Rocks Rd, Nelson) – Road open
Canterbury (including alpine/mountain passes):
• Inland Route 70 (Inland Kaikoura Rd) - Road closed (flooding)
• SH1 Hundalee – Road closed between Hundalee and Oaro township (slip)
• SH1, north of Kaikoura, Mangamaunu to Clarence – Road closed
• SH1, south of Kaikoura, between Peketa and Goose Bay - Road closed
• SH6 Hokitika to Haast - Road closed (strong winds)
• SH73 Jacksons to Kumara Junction – Road closed (strong winds)
• SH6 Westport to Greymouth (Crossroads to Rapahoe) – Road closed (strong winds and flooding)
• SH6 Inangahua Junction to Wesport (Lower Buller Gorge) – Road closed (fallen trees)
• SH6 Haast to Makarora (Haast Pass) – Road closed (strong winds)
• SH6 Haast to Kingston – Care needed (flooding and rockfalls)
• SH8 Alexandra to Milton – Under stop/go control near the intersection with Grovers Hill Rd
• SH8 Lindis Pass to Raes Junction – Care needed (flooding and rockfalls)
• SH85 Kyeburn to Alexandra – Care needed (flooding and rockfalls)