Investors in a retirement trust fund run by insurer AMP were surprised to find their balances had doubled overnight - then dismayed to find it was merely a mistake.

The company revealed the error in an email to 1500 customers this week, two days after its monthly update was sent to investors in its New Zealand Retirement Trust scheme with the inflated balances.

It said there was a "technical glitch" at fault for the higher figures reported in its January missive, and its team was actively working to fix the issue.

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Auckland man Chris Wilks was one of the customers affected, and said he had been extremely pleased to see his $36,000 from January last year was up to $76,800.

"I very rarely look at the emails but I just happened to glance at it and notice it had doubled," he told the Herald.

However, he wasn't surprised to receive the next email from AMP saying "we made a mistake".

"It just goes to show that if something seems too good to be true it usually is."

Wilks wondered if the provider had possibly been hacked, or the victim of fraud.

However, in responding to questions, AMP said it was an issue in the design and production of the email newsletter only and not with any of its core systems or data.

"At all times, clients' funds were safe and they could view their correct account balance through our My AMP app and portal," it said.

"Unfortunately, sometimes mistakes do happen, but we have taken steps to prevent this from happening again."


Wilks said while that was a relief, it would be more reassuring if the provider didn't make mistakes.

"I just hope they don't do it again."

AMP said it had emailed and phoned clients to apologise.