The reopening of State Highway 1 over the Brynderwyn Hills will be delayed by a number of weeks due to an active slip.

The section of highway was damaged during last year's Cyclone Gabrielle, and had been closed for repairs since mid-February.

The critical Northland highway was due to fully reopen on the 13 May.

NZTA Waka Kotahi says there was a slip over the weekend and it has continued moving over the last couple of days.

A substantial amount of material fell on Tuesday, it said.

The agency said it was still in the early stages of assessing the best solution, and would confirm more details as soon as it could.

NZTA Waka Kotahi spokesperson Mark Kinvig told Checkpoint the sheer volume of material that fell had caused the delay.

The material is highly variable, suspeptible to slips and "is quite a tricky site to work through".

The priority was safety of the workforce in the next 24 hours, Kinvig said.