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The collapse of earth under State Highway One means the road will be closed for several days at the Brynderwyn Hills in Northland.
Heavy rain in Northland last night led to a 10m-long section of hill collapsing under the highway this afternoon, the NZ Transport Agency (NZTA) said.
"Unlike the underslip on SH1 at Kawakawa last week, there is no room on the Brynderwyns to build a detour route around the collapsed section of road. We will have to repair the underslip itself before we can re-open the Brynderwyns," said the NZTA's Regional Journey Manager Jacqui Hori-Hoult.
The slip was still moving and a detailed assessment of ground stability would have to be completed before repair work could begin, she said.
"We are aware of the importance of this section of highway as the main road link between Northland and Auckland. Everything will be done to have it repaired as quickly as possible, but it will only be re-opened when we are completely satisfied it is safe for people to use."
Two detours are available for drivers. One is via SH12 through Paparoa and Oakleigh, and the other is via Waiapu and Mangawhai. The detours will add another 30 to 40 minutes to journeys, NZTA said.
"An underslip can be more a challenge to repair than a part of hill that has fallen on top of a road," Ms Hori-Hoult said.
Heavy rain in Northland also caused surface flooding in many areas. People were asked to drive to the conditions by lowering their speed, increasing their following distance and allowing more time for travel, Ms Hori-Hoult said.
Northland Civil Defence spokesman Graeme MacDonald said the rain caused a number of issues, including slips and flooding overnight.
Northland Regional Council rainfall figures for the 24 hours to 6am today showed more than 100mm of rain fell in some areas in Northland.
A peak rainfall intensity of 33mm/hr was recorded at the Brynderwyn rain gauge early today, Mr MacDonald said.
River levels throughout Northland were monitored closely and although they had risen significantly overnight, causing localised flooding in some areas, there were no widespread issues.
Anyone whose home was flooded or who urgently needed accommodation, food, heating, clothing or medical supplies should contact their local council in the first instance, Mr MacDonald said.
Meanwhile, a heavy rain warning for the region issued by MetService has now been lifted. The rain was caused by a low which was forecast to move slowly away to the north-east today.
Further showers can be expected as the low moves away, with about 10 to 15mm possible in south-eastern areas, but the most significant rain has now ceased.
- Brendan Manning and Rebecca Quilliam of APNZ