State Highway 29 over the Lower Kaimai ranges near Tauranga is closed due to the risk of rocks falling on road.

Both lanes are closed about 17km west of Tauranga near Ruahihi Road because rocks, which may have been dislodged in heavy rain overnight, could fall onto the road, the New Zealand Transport Agency (NZTA) advises.

"Rocks on the Ruahihi Bluffs have been undermined and now pose a potential risk to motorists," NZTA spokesman Brett Gliddon said.

"As always, we will do our best to keep delays to minimum and get the road open as quickly as possible, but we can't predict how long the road will remain closed at this time."

Motorists travelling between Tauranga and Auckland or Tauranga and upper Waikato should take State Highway 2. While those travelling between Tauranga and lower Waikato should take State Highway 36 and State Highway 5.

Police also said part of State Highway 10 near Kaeo in Northland is down to one lane and is closed to trucks due to a slip.