The woman said she gave her contact details for the register, but there was no information about them being passed on, or an opt-out option.
"I think it's not right; you should have the option," she said.
She complained to the real estate agency's branch manager and the company told the Herald it received four separate complaints from house-hunters affected by the lapse.
LJ Hooker Blockhouse Bay branch manager Susan Podwin said the business was reviewing the way names were put into the open home register.
The finance company, Austar Financial Services, was run by the same managing director as the real estate branch and the complaints stemmed from an agent who entered people's details directly into a company database on his iPad.
"Somehow when their names go on to our open home register [they have slipped] straight through to the financial services," Ms Podwin said.
Clients were usually given the option to opt out, she said.
In another case, an Auckland man received pamphlets and vouchers from a shopping mall two days after he moved into his new Mt Wellington house, which he bought through a different real estate company.
They were addressed to him using a nickname that he said only people who knew him personally used.
"The only way they could know that I'd purchased a house, my name, and my address, would have been through the sales agent," he said.
The agent denied his details had been passed on to a third party.
Charles Mabbett, spokesman for the Office of the Privacy Commissioner, said the commission had had inquiries about similar potential privacy breaches from house hunters.
"We understand the need for real estate agents to record the contact details of prospective house-buyers who visit the property," the spokesman said.
"But it should not extend beyond that, unless the person is given a choice to opt in to receive marketing information."
He said he was unclear whether the cases were in breach of the Privacy Act, as decisions were made on a "case by case basis" and a complaint needed to be lodged first.
"If someone suspects that their information has been passed on to a third party without their consent, they can make a complaint to us.
"If we receive enough similar complaints, we may then work with an industry sector to get them to improve their practices."
A spokesman for the Real Estate Agents Authority said he believed the practice was a breach of the Privacy Act but could not recall any complaints about similar breaches.
Real Estate Institute chief executive Helen O'Sullivan said it was important agents ensured people agreed that their details could be passed on to third parties.
Kath Dewar, managing director of marketing company GoodSense, said: "Unless the register said 'we're going to use it to market out other products and services', then they shouldn't be using it to market them."
