Sarah Healey and her partner, Eddie, were about to sign for a loan on their Grey Lynn home when they got a surprise from the past.

It was 2006 and before they could secure a mortgage, for their three-bedroom home, Ms Healey's bank did credit checks on the design tutor and her partner, an online journalist.

"I apparently had an outstanding power bill from my last student flat in Dunedin that was for about 100 bucks or something," said Ms Healey, 31.

The problem was that she had moved out of the flat she shared with five others while studying design at Otago University - in 2001.

"The bank went digging around in my credit history and there was a black mark that showed up.

"I promptly got a letter from Baycorp saying something like, 'You owe TrustPower about 100 bucks'. Once I paid it, it [the loan deal] went through, no problems at all," she said.

Ms Healey is one of the more than 800,000 New Zealanders who have a black mark against their credit histories that many are unaware of.

She left Dunedin after graduating in 2001, not realising the bill at her old flat was still in her name.

Before applying for her home loan, she had never had a hire-purchase agreement or a credit card.

She said the bill wasn't a problem but it was a shock to see something come back from the past.

"There were so many people unaware of the debt, so surely it would suggest there is a problem with the way power companies are communicating with their customers," Ms Healey said.

"If they wanted me to pay it, why didn't they contact me?

"Perhaps they sent mail to my Dunedin address, but I never got any and maybe my untrustworthy flatmates never forwarded any on."