The wash-out on State Highway 25 on the Coromandel Peninsula has widened, but essential light traffic is still able to use the route.
Heavy vehicles are prohibited.
Heavy rain on Sunday night forced the closure of the road between Opoutere and Hikuai, north of Whangamatā until Monday afternoon when one-lane was opened under traffic management.
Waka Kotahi said contractors were cutting into the bank to extend the road further away from the advancing underslip.
It said they were keeping traffic flowing as much as possible, but motorists should expect long delays.
The transport agency said ground conditions in the area remained highly unstable and it was constantly monitoring the site to ensure it remained safe.
The road was closed at 6pm on Tuesday for an assessment on Wednesday about a repair plan.
Thames-Coromandel Civil Defence controller Garry Towler told Checkpoint contractors were hoping the remedial work done on Tuesday would hold up until then.
"I can tell you right now these contractors up there right now are absolutely hell-bent on keeping this open ... there's a great resolve by the contractors to win this battle and fingers crossed they will."
They did not know if the road's condition would become worse overnight, or how long a repair would take, but motorists were being kept away from the vulnerable areas, Towler said.
"Hopefully that [work on Tuesday] will maintain itself and hopefully not provide too much vibration which will cause the rest of the road to go."
The road was a lifeline for the area and losing it would spell trouble, he said.
"We were fragile to the core anyway, [if the road completely goes, it] just means that the only road available for the supply chain onto the Coromandel will be via the Thames up to Coromandel then the cross over to Matarangi and Whitianga.
"And that road in itself is quite fragile as well ... there's continuous rock fall, debris fall and the road is fragile and all the all the banks and all the hillsides around it as a result of this, you know, just week after week of severe storms.
"This is unheard of, we've had 2.3 odd metres of rain this year alone. We don't normally get that rain until October."
RNZ
