By the time a local emergency was declared in Hawke's Bay people had already died and hundreds were stranded on rooftops hoping to be rescued

A Hawke's Bay Regional Council hydrologist says it is unlikely the magnitude of the flooding during Cyclone Gabrielle could have been accurately forecast by the council.

Hydrologist Craig Goodier was giving evidence at a Coroner's inquest on Monday into the deaths of 13 people in relation to the storms.

Goodier was the council's flood forecaster during Cyclone Gabrielle. In this role he had to determine the impacts of the flood, and identify areas of concern in the region.

In his evidence, Goodier explained how the cyclone was expected to be similar to the 2018 flood in Esk Valley.

In that flood, several roads were closed, 12 farms, orchards and vineyards were damaged and there were no reported injuries or deaths.

"But, the peak discharge of water in the Esk River has now been estimated to be more than double 2018's," he said.

Goodier said all flood models contain various levels of uncertainty and the largest item that affected the model results was the rainfall input.

"I believe that it is unlikely that that magnitude of flood could have been accurately forecast by HBRC. This is because of the very large difference between the forecast rainfall and the actual rainfall in some locations."

The actual rainfall recorded in some areas was 650 millimetres in 24 hours, about twice as much as what was forecast in some locations.

Further south, the Tutaekuri River and the Ngaruroro River stopbanks were designed to withstand a 100-year flood. In the cyclone there were multiple breaches of stopbanks, and six major bridges were destroyed.

"On the basis of the forecast rainfall data from MetService, I had no expectation that the stopbanks would be breached," Goodier said.

The Coroner's lawyer, Mathew Mortimer-Wang, questioned Goodier about whether he could have better communicated the risk, in relation to information he provided authorities about Eskdale.

"In hindsight it looks a bit light. It could have been better worded, but we are on time schedules. I thought it was appropriate at the time," Goodier said.

Earlier in the morning, the hydrologist explained his flood forecaster role was limited.

"I did not, for example, make decisions about evacuations or declaring a state of emergency - those were clearly civil defence matters outside the scope of HBRC's functions; provide weather forecasting or issue weather watches or warnings to the public. That role was carried out by MetService."

In Goodier's witness statement, he said the debris in the flood waters likely contributed to the blockage at the mouth of the Esk River.

"It would appear that on the night of 13-14 February a raft of logs and other debris blocked the Esk River mouth and caused the water level upstream of the blockage to rise significantly.

"I did not personally observe the blockage, but I am working on this assumption."

Two people drowned in Eskdale during the cyclone. Susane Caccioppoli and Gareth Jones were trapped in a ceiling cavity in Esk Valley, when a large object carried by floodwaters struck the house, and they were flung into the torrent, where she died.

Nearby, two-year-old Ivy Collins was swept into the floodwaters as her family tried to escape.

Goodier showed the court an image of the Esk River mouth, which appeared to illustrate a pile of logs nearby.

"In my opinion, given the rainfall that had been forecast, a log raft blockage at the Esk River mouth on the night of 13-14 February 2023 was not foreseeable," he said.

Goodier said that's because in the 2018 flood there were no issues with log jams at the river mouth, and it is hard to predict how much debris will wash down into the river from the catchment area.

Evacuations for Eskdale didn't begin until nearly midnight on 13 February, and even then it was only voluntary as firefighters were knocking on doors asking people to evacuate in the Shaw Rd area.

An official emergency wasn't declared until 4.04am on 14 February, and an Emergency Mobile Alert wasn't issued until 5.19am. By which time two people had already drowned.

But in catchment summaries provided to HBRC, the flood risk in Esk Valley was singled out two days earlier.

"Consider vacating Esk holiday park and any other at risk areas," it read.

The next morning, an operational meeting with Chris Dolley, the HBRC flood modeller, and others said: "Esk is big concern".

Several hours later, a catchment by catchment summary singles out part of Esk Valley.

"Houses on Shaw Rd may be at risk. Could be similar to March 2018. Campgrounds have already been evacuated".

Goodier was questioned about the delay of an important river catchment summary later on Monday 13 February, as it wasn't issued until 11.30pm, more than two hours after Metservice's updated forecast at 9.05pm.

Between that time he said the river levels were rapidly rising, so the HBRC flood modeller took his time to manually look at each site of concern.

"Because there are so many sites at this time that were alarming, it did take him quite a while," he said.

"He got to the point where he thought he better get this information out, which we agree was delayed, but that was as quick as he could do and he wanted to be thorough."

There was also a delay on accessing some rainfall data, a power outage meant many of the regional council's rainfall stations did not upload their data. Goodier said this meant their system was showing incorrect rainfall totals.

"Through the night, the physical destruction of some water level recorders and inundation of some rainfall gauges compounded the difficulty of providing accurate actual information. However, I was not aware of any of these failures over this time."

Data after the cyclone showed some areas in the Esk River catchment recorded 590mm, and one site in the Ngaruroro recorded 649mm in 24 hours.

Goodier was also questioned about a report from the regional council following a big flood in Esk Valley in 2018.

At that time, Esk Valley Campground Owner Dan Gale asked the council to investigate a flood warning system that was linked to 'trigger levels' in the river, so that residents were evacuated when the river reaches a certain height.

Goodier said the proposal may have been lost in a "bunch of other stuff" from that time.

"I think it's a good idea and we are following up on that now."

The communications between Metservice and the council was in the spotlight at the inquest, in relation to when the weather warnings were upgraded.

MetService chief meteorologist Chris Noble told the court there were "opposing views" on the need to escalate to a red warning.

Goodier said he was not aware of any requirement MetService had to engage with HBRC, and in particular, himself, before they would go to a red warning.

"Nor was I aware that MetService was placing any particular stock on my technical response to their question in making that decision.

"From my perspective, it was MetService's job to make the decisions on the warnings using their expertise about forecasting the weather."

However, Noble said while it was MetService's role to issue and upgrade weather warnings, it was typical to discuss these decisions with councils, and factor in their local expert information.

He said this collaboration could vary by region.

Cyclone Gabrielle inquest: 'Unlikely' the magnitude of flooding could be accurately forecast, expert says