The cost of dealing with ex-Cyclone Gita has been put at $4.5 million, according to a new report into the crisis that makes six major recommendations for improvements.

A state of emergency was declared after the weather system slammed into Taranaki on February 20, bringing down power lines, closing roads, and leaving 10,000 homes without water for three days after a tree fell on a water main near the water treatment plant south of New Plymouth.

Some 26,000 properties were under a boil water notice for seven days afterwards, while the incident controller and operations manager worked over 100 hours each in the first week.

ANDY JACKSON/STUFF A broken pipeline caused by cyclone Gita has cut water supply of to parts of north Taranaki.

A long-awaited report into the New Plymouth District Council's performance during the crisis was finally made available to the public on Wednesday and gave the local authority's response a pass mark.

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The 57-page report, written by council infrastructure manager David Langford and peer reviewed by Wellington Water's Ian McSherry, had been due to be completed at the end of May and then released to the public.

ANDY JACKSON/STUFF Businesses had to close as the water shortage stretched to three days and the boil water notice for a week.

It makes six 'significant recommendations', including looking at budgets, infrastructure, raw water sources for contingency use, risk assessment of water connections, asset management and an update to emergency plans, as well as 22 minor points.

The report looked at the tree that fell on the water main, causing the most significant loss of supply the district has ever experienced, and found that had it fallen at a slightly different angle the pipe might not have ruptured.

"Whilst the risk of trees falling onto the pipe bridge was present, it was not an unidentified risk and controls were in place... The bridge structure forms a triangular prism-shaped cage around the pipe to protect it from impact.

ANDY JACKSON/STUFF The storm hit on Tuesday, Feb 20 and by 8pm a state of emergency had been declared.

"Had the tree fallen at a more obtuse angle and glanced the bridge it is possible the A-frame would have shed the tree and minimised any damage to the pipe."

The issue of critical spare parts was also highlighted. An asbestos cement replacement for the central feeder main pipe was kept at the water treatment plant but had been in storage for so long it was unusable and not "able to self-support with the damaged/temporarily repaired bridge".

Had the pipe been usable the repairs could have been completed four hours sooner, but the report said it was highly unlikely this would have made a "material difference to the outcome of this incident".

ANDY JACKSON/STUFF New Plymouth begins to feel the wrath of Cyclone Gita.

Afterwards arborists surveyed all trees within 'fall range' of critical water and wastewater mains. In total, over 10,000 trees were assessed, with 13 needing to be felled due to their condition, with five being a fall risk.

All NPDC pipe bridges are inspected annually but future inspections will also report any hazards in the surrounding area.

The economic impact of the water supply interruption is estimated at about $4.5 million. This including $580,000 of council expenditure and staff time, and $3.9m in businesses, healthcare workers and large water users closing down or operating in limited capacity, as well as the non-productive time of working parents having to arrange childcare.

ANDY JACKSON/STUFF Civil Defence Minister Kris Faafoi took a tour through the Taranaki Civil defence base in New Plymouth.

"There were no reports of illness due to waterborne disease, roads were quickly cleared and reopened, there were no sewage overflows, water supply was restored to all properties (residential and commercial) within three days and full compliance with the drinking water standards was restored within 10 days," the report said.

Feedback was sought from businesses, other councils and government agencies, emergency services and members of the public.

Strengths included the fast response to repair the water main, the leadership of the Emergency Management Team and communication during the crisis.

But the report called for improvements to the cramped control room, the sourcing and coordinating of volunteers and non-internet communication.

A lack of spare equipment and staff resourcing was also highlighted. "It is probable that the current resources in place would struggle to provide an effective response should there be an event much larger or longer in duration, such as a large magnitude earthquake or volcanic eruption.

"With so little depth or redundancy within the team, key individuals were required to work excessively long hours (up to 18hrs at a time) without rest."

Another issue was the lack of preparedness of the community, despite warnings from Taranaki Civil Defence for three days before the storm.

Gita also highlighted the fact that NPDC's drinking water, wastewater, stormwater, roads and the airport are all interconnected.

It was recommended that valves that can be shut remotely or shut automatically in an earthquake to prevent water escaping be installed.

Emergency backup generators at key sites were also recommended.

Ex-Cyclone Gita timeline:

February 19, 2018: Warnings are issued for ex-cyclone Gita, which approaches New Zealand from the west.

February 20: Rain starts falling in central NZ in the early hours of the morning and the weather worsens throughout the day. Roads are closed in several areas due to flooding.

Feb 20, 3.25pm: Wind gusts knock over a tree, which falls on and breaks the water main.

Feb 20, 3.30pm: Powercuts start to occur.

Feb 20, 4pm: Emergency response established.

Feb 20, 8pm: Communication on saving water is sent out and a state of emergency is declared.

Feb 20, overnight: 16 roads closed due to falling trees or land slips, and high winds break windows at the water treatment plant.

Feb 21, from 5am: High water users and businesses urged to shut down non-essential services.

Feb 21, 8.30am: Hairdressers advised to close due to the chance of chemical backflow and customers being left mid-treatment. A water tanker is connected to the airport supply so they can continue to operate. The replacement pipe is dispatched from Auckland.

Feb 21, 9am: Water tankers are refilled from the Inglewood supply and deployed across the affected areas of the city.

Feb 21, 11.30am: Boil water notice issued

Feb 21, 6pm: Pipe arrives on site. Work is suspended due to heavy rain at 11pm.

Feb 22, afternoon: All slips and trees blocking roads cleared.

Feb 22, 7pm: Pipe bridge and water main repairs completed. The refill and cleansing process begins at 8pm.

Feb 22-24: Refilling, flushing and monitoring of water supply continues.

Feb 25, 11am: State of emergency lifted.

Feb 26 - Mar 1: Water quality testing continues. At 10pm, final test results confirm compliance with drinking water standards.

Mar 2, 11am: Boil water notice lifted.

Mar 2, 12pm: The Emergency Control Room and Emergency Responses Team is disestablished.