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A computer system upgrade has caused delays for customers trying to reach Contact Energy's call centre.
The electricity generator and retailer said work over Easter to combine 20 existing computer systems into one integrated system had caused the problem.
Spokesman Shaun Jones said the issue was compounded by an increase in calls relating to power cuts after Easter storms.
The company's 240 contact centre staff were still coming to grips with the new system.
"When you adopt a new system it takes time for people to get up to the speed they were on the previous system," he said.
Staff were working "around the clock" to catch up with unanswered queries.
Mr Jones apologised to customers affected by the delays.
"We understand the frustration our customers have been feeling and we apologise for the delays in answering their calls."
On Contact Energy's Facebook page, angry customers have posted comments about long waits to reach the call centre and being cut off while on hold.
Others complained they could no longer access their accounts online.
One post read: "Why has your customer service gone from excellent to nonexistent! Your phone line today started with a 40 minute wait to then sorry are lines to busy call back later?! (sic)"
Yesterday, another customer wrote: "I understood when I called contact tonight that there were long waits for the call to be answered which I was fine with but for the call to be disconnected after waiting exactly one hour is terrible."
Mr Jones said no customers would be out of pocket as a result of the delays, and would not lose their prompt payment discount if they payment was delayed through no fault of their own.
Contact has more than 560,000 customers around the country.