NZ Transport Agency system manager Steve Mutton said early assessment by engineers was that it could take at least two weeks to make the hillside safe enough to re-open the road to traffic.
"A lot depends on the weather. It's fine right now, but there have been days of heavy rain and more is forecast next week. More rain could potentially see the slip start to move again."
Initial work to clear and stabilise the slip had started. Engineers were using a drone to help survey its extent.
All other state highways had re-opened but NZTA crews were still checking for damage caused by surface flooding.
Muriwhenua area fire commander Wipari Henwood said given the water table is already full, the fire service is on high alert for the possibility the cyclone comes down.
Spray flies as a four-wheel-drive negotiated a flooded Pupuke-Mangapa Rd north of Kaeo yesterday. Photo / Peter de Graaf.
"We're watching the weather modelling very closely with Civil Defence."
Extra fire crews were brought in from Auckland on Tuesday afternoon, after fears an incoming tide could cause flooding at the bottom of Turntable Hill and Kawakawa's Three Bridges where water was lapping at the road at 4pm.
However, crews were stood down at 8pm after high tide passed with no further flooding.
Niwa principal scientist Chris Brandolino said it was "way too early to speculate" on what effect, if any, Cyclone Gita could have on Northland.
However, he said if rain did come, there would be a higher risk of flooding across much of the North Island, due to the recent rainfall.
Flooded farmland beside Pupuke-Mangapa Rd, north of Kaeo. Photo / Peter de Graaf
Mr Brandolino said the cyclone is south west of Fiji and likely to keep heading west towards New Caledonia.
"On Friday it's likely to slow down and turn more southern. Then we'll have to watch it."
He said it would likely be Monday or Tuesday when any remnants reach New Zealand.
Meanwhile, heavy rain has forced the cancellation of this Saturday's 104th North Hokianga A&P show at Broadwood.
