A falling tree which killed a Rotorua woman was a "catastrophic failure" during an extreme weather event, the council says.

Documents released by the Rotorua Lakes Council under the Local Government Official Information and Meetings Act (LGOIMA) detailed the condition of the tree which fell and crushed Trish Butterworth during a storm on January 5.

Council acting operations group manager Henry Weston said January's weather event was a "hell of a storm" that was "pretty unusual" for Rotorua.

The incident was a "catastrophic failure", he said.

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Weston said the "tolerable" level of risk identified in an arborist's report in February 2017 related to the risk of a tree limb, or branch failure.

No structural risk was identified.

Contractors clean up the large oak tree which came down during the storm in Rotorua, killing one woman and damaging a building. Photo / Ben Fraser
Contractors clean up the large oak tree which came down during the storm in Rotorua, killing one woman and damaging a building. Photo / Ben Fraser

Tolerable risk meant the risk of harm was low to very low. The arborist noted there was one 100mm piece of deadwood on the tree. The removal of the deadwood would reduce the risk to broadly acceptable, the report stated.

Concerns raised with the council in August led to braces being replaced and weight reduction work taking place.

The braces wrap around the tree to provide extra strengthening and lock the limbs together, reducing the risk of limb failure.

Weston said while the tree had heritage value, the council would never have put the tree before public safety.

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The council had gone through all its files relating to the tree and handed them over to police, he said.

Police confirmed on Monday their investigation on behalf of the Coroner was ongoing.

The documents also showed concrete had been poured into the tree.

Weston said pouring concrete into a tree had been common practice in the past, but as time went on there was "general consensus that it just wasn't effective".

The council acquired the tree in 1991, after the owners of the land it stood on decided they no longer wanted it.

The tree was of historical significance so the council was obliged to acquire the land to ensure it survived.

It was believed the concrete was poured before the council took over ownership of the tree, so it was unclear when it was put in and how much of it was used, Weston said.