Homeowners are being warned to prepare for a devastating interest rate rise that could see some lose their properties.

The Reserve Bank of New Zealand has warned people to prepare for a potential 2 per cent increase from 2014 to 2016, following years of low interest rates.

An announcement will be made on December 12, three months after the Reserve Bank's half-yearly Financial Stability Report revealed the expected rise.

With average floating mortgage rates sitting at about 5.8 per cent, a 2 percentage-point lift would see mortgage rates lift to nearly 8 per cent, some economists are predicting.

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A Reserve Bank spokesperson refused to comment on the expected increase but economists and other financial experts are already warning people to prepare, saying they are becoming too complacent after years of low interest mortgages.

Tauranga mortgage broker Chris Rapson said the proposed increase would be devastating for Bay of Plenty home owners.

"If you have a $300,000 mortgage and the Official Cash Rate was to move by quarter of a per cent in a year, it's $750 a year . . . So now you have to earn $1000 more.

"That's quite a lot of money," Mr Rapson said.

"I see people every day looking to get a mortgage and I know how tight their statements are."

Mr Rapson said this could result in more mortgagee sales and a decline in house prices.

BNZ chief economist Tony Alexander said it was possible floating mortgage rates might rise to 6.8 per cent by the end of next year.

Tauranga Budget Advisory Service's Diane Bruin said some people would have to give up their homes.

"Previously a lot of people have purchased property with less than 20 per cent deposit and this will now increase their repayment amounts to unrealistic repayment rates."

Tauranga broker Alan Hitchcock said some home owners would lose out.

"I have a feeling they could be caught out.

"There are still a number of people I talk to sitting on a floating interest rate and unaware that this is going to move," he said.

Fellow mortgage broker Frank McLean said he doubted that the increase would be as high as that predicted. "The thing is, no one really knows what's going to happen. Even the Reserve Bank does not really know."

Tauranga woman Meg Lomas bought a home two years ago with a mortgage of about $400,000. She said the extra 2 per cent would work out to be another $80 per week for her family.

"That is a lot of money," she said.

Greerton property developer Bill Shaker, 59, can still remember paying around 22 per cent on his mortgage in the late 1980s.

"It scarred me," he said.

"People have got used to a semi-fixed interest amount and many people have gone out and mortgaged themselves up to their eyeballs, they simply can't afford to take on any more increases, but they will have to.

"You've got a whole group of people who think these new interest rates are the new normal but they are not."