The detour can be accessed from the SH1/SH12 intersection at the Brynderwyn Hills, or from SH14 between Whangarei and Dargaville.
It may add up to 2 hours to journey times.
Ms Hori-Hoult said work to widen SH12 at Kaihu, 30 kilometres north of Dargaville, will be completed today.
A 70 metre-long section of the highway was washed away by flood waters.
People using Ruapekapeka Road as a detour are reminded it is unsealed and winding, and extreme care is needed.
It is open only to light vehicles, and there is no access for heavy vehicles.
Mangakahia Road remains closed to heavy vehicles north of Twin Bridges.
South of Whangarei, crews are busy repairing potholes and clearing slips and drains.