NZTA said a crew had inspected the culvert under the road an hour before the washout and there was no sign it was blocked or that there were any issues at that time.
Since Friday the weather has improved and the water level in the stream had dropped.
NZTA Northland system manager Jacqui Hori-Hoult said it was working on opening the road to one lane traffic as soon as possible but that was unlikely to be before the middle of next week.
While SH1 was closed there was no appropriate alternative route between Kaitaia and Cape Reinga.
Motorists should avoid the area and delay their journeys.
NZTA said some locals had used a private track through the forest to get around the washout.
But the Transport Agency, Police, Civil Defence and local authorities reiterated this route was a private unsealed one lane track that would have deteriorated with recent rainfall and use over the weekend.
Hori-Hoult said the forestry track was available for emergency use only and was not safe for other vehicles.
"The route is privately owned by Summit Forestry and from 4am Monday morning it will become operational and is only to be used for emergency access.
"That access will be actively monitored by police."
Driving on 90 Mile Beach was also not advised where the sand and tide present safety hazards to people not familiar with driving in these conditions.
Civil Defence said there had been no requests for assistance north of the slip and police were on standby to deal with emergencies.
NZTA said the washout repair has not taken resources from the Lemons Hill slip repair work on SH11 between Paihia and Kawakawa. That work was continuing.
