Massive slips - some up to 100m high and 50m wide - have been discovered along the Utuhina Stream.

Back-to-back ex-tropical cyclones Debbie and Cook, and ongoing rain, created the perfect environment for the slips to occur at the Utuhina Stream headwaters.

Natural land dams were also formed and further rain carried the debris and soil downstream.

The slips have caused the Utuhina Stream to appear 'milky' which, according to Bay of Plenty Regional Council science manager Rob Donald , is perfectly safe for fishing and swimming and is a result of fine white material in the soil.

"The soil that caused the slips is from old lake bed sediments which formed thousands of years ago as deposits of pumice, organic material and diatoms (algae) on the bed of Lake Rotorua.

"The sediments are extremely fine and are present as layers up to two meters thick in the area - it can become unstable and slippery when saturated and in this case it is near a natural spring which is also running milky," he said.

"It's a soil type that is susceptible to slips and the deluge of rain we saw over March and April put it to the test. The damage to the earth shows just how powerful mother nature can be."

Lake Rotorua was formed during a single eruptive event around 240,000 years ago and was about 130m higher than the lake level we see today.

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As the water level dropped it left behind the soil layers that we see around the lake today.

"Our city has been built on this dried-up lake bed, so there are large deposits of this soil type across Rotorua, and this 'milkiness' we're seeing in Utuhina Stream will be present for some weeks to come," said Dr Donald.

Bay of Plenty Regional Council is currently undertaking 3D modelling of the slips and resulting natural land dams to ascertain whether any remedial work will be required.