An oak tree toppled during a storm in January was described as being of a "tolerable" level of risk of dead limb failure, months before it killed a Rotorua woman.

Documents released by the Rotorua Lakes Council under the Local Government Official Information and Meetings Act (LGOIMA) show an independent arborist assessed the tree in February 2017 and found it was showing "signs of decline" but was not a high risk.

But concerns raised with the council in August led to braces being replaced and weight reduction work taking place in September and October.

A few months later, Trish Butterworth, 56, was killed when the tree fell and crushed her during a storm on January 5.

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Spencers Oak in Rotorua before it fell in a January storm, killing Trish Butterworth. Photo / Google Maps
Spencers Oak in Rotorua before it fell in a January storm, killing Trish Butterworth. Photo / Google Maps

The tree, known as Spencer's Oak, split in two during the storm, falling on to the Te Arawa House building and across the road, crushing a car Butterworth was driving. She died at the scene.

Police say they're still working on their investigation into the incident on behalf of the coroner.

The just-released documents also reveal concrete was poured into the tree.

The council acquired the tree in 1991, after the owners of the land it stood on decided they no longer wanted to own it. The tree was of historical significance so the council was obliged to acquire the land to ensure it survived.

The tree was planted in 1863 from one of four acorns Queen Victoria sent to Reverend Richard Taylor.

In March 2002, Wakeling & Associates presented a tree report to the council.

The report found the tree was mostly in good health.

But it did identify a "large cavity" in the centre of the main fork.

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"Since that time the tree has formed a sound ring of healthy wood around the outside, but has not fully occluded [closed up]– in fact the wound will never heal over," the report stated.

The cavity that formed below that wound was "very deep", and had been filled with concrete and pumice.

The surface of the concrete was about 600mm below the opening.

"The internal part of the cavity extends into the heartwood of the main leaders. I am unsure as to how far this decay has spread up the leaders. The width of the cavity inside the tree exceeds 600mm," the report stated.

The trunk was found to be in a stable condition.

The report recommended further investigation of the fungal bracket and the extent of decay near the base of the tree.

In February 2017, arborist company Arbor Care presented a visual tree assessment of the oak.

It found there were "several old wire rope cables" installed in the tree, some of which appeared to be "under a lot of tension (one cable is frayed and is unravelling, and one has a broken ring)".

Although the cables appeared to be under tension, there were no signs the tree needed them.

"This tree appears to be showing signs of decline, the crown is somewhat thin for this time of year and the foliage is not the usual dark green but is showing signs of yellowing," the assessment stated.

It detailed three possible risk categories - broadly acceptable, tolerable, and unacceptable.

There was a "tolerable" level of risk of a tree limb, or branch failure.

No structural risk was identified. The removal of deadwood would reduce the risk to broadly acceptable, the assessment stated.

According to a report released under the LGOIMA, in late August council open space operation adviser (horticulture) Mark Paget met with arborist Thomas Rika, where he raised concerns that one of the oak's bracing cables was giving way.

"As a result of this discussion I went and inspected the Arawa oak and I immediately arranged for Treescape to put up a temporary brace and Infracore installed a second temporary brace the following week," Paget says in the report.

"We then contracted Treescape to install permanent braces and do some weight reduction work."

Upon inspection in early October, Paget observed that one cobra brace had been installed to replace the fraying cable brace, and an additional cobra brace had been installed to assist another existing brace.

The council has been approached for comment.