The West Coast Regional Council is scrambling to understand how it sent out wrongly calculated rates bills.
Council officials again apologised on Thursday for the mess, acknowledging it was "not good enough".
On 27 June, council approved a 16.9 percent general rates rise for 2023-24.
But ratepayers who received their first instalment invoices last week were shocked to discover increases of anywhere between 64 percent and 100 percent.
The council apologised on Monday for the "potential" glitch after the problem emerged late last week.
On Thursday, council chairman Peter Haddock they had engaged an external auditor to trawl through the rates accounts - to get a handle on the extent of the problem, and what to do next.
"Hopefully by today we'll have the professionals confirm what it is. I apologise on behalf of the council," he said.
Chief executive Darryl Lew said it was too early to say how many ratepayers were affected and the extent of the problem across the regional council's 15,000-odd rates base.
"That work is underway, and it's happening," he said.
He would not be drawn on the question of whether people had the right to demand a refund from council.
"I'm checking very carefully all the demands that have gone out against the rates that council set. I want to do some very careful checking and get it right."
Lew acknowledged ratepayers would be reeling and unhappy at what appeared to be a mistake at council's end.
"No, it's not good enough and there needs to be an internal review. I'm working through the extent of the problem - it's not good enough."
It was still not clear outside Grey District the extent of incorrectly invoiced properties in Buller and Westland, and by how much.
"What we're hearing from ratepayers is people ... have had significant increases in rates - more than what they were expecting, and more than council passed," Lew said.
Haddock said he became aware of the issue when contacted by a ratepayer on 27 September and subsequently last weekend looked at the rates for properties he owned.
"I realised there was something wrong … I discovered myself some errors."
Haddock said he was reluctant to offer an opinion on what caused the problem until the independent audit reported back to council.
"I think I know what's wrong, but it's not for me to say … They've brought someone in to cast an eye over it … I believe that the figures that were done for council for the annual plan were correct."
However, Haddock said he had looked at the rates struck by council on 27 June and believed what had been passed at the time "was correct and the right factors were used".
"Somewhere along the line (the system) sent out the rates demands with the wrong figures.
"It's an embarrassment for council, that's for sure."
Haddock said he had checked if the correct QV rates valuation settings had been applied for individual properties, depending where they were.
"Some people who paid their rates in full, there will have to be a refund I would imagine.
"Obviously it will vary on different properties depending on the valuations. It's an embarrassment council don't need.
"I think the good news of the story as reported in the paper, there won't be a 100 percent increase."
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