Home owners who think they have escaped the leaky building crisis could be in for a nasty shock.

Louise Cosgriff has a message for the thousands of modern home owners who still think their house won't leak - check it now, before it's too late.

The leaky homes consultant, who worked on an earlier report gauging the size of the problem for Auckland City Council, said she believed building experts were spot on with their prediction to the Government that 89,000 homes built between 1992 and 2005 would leak at a cost of $23 billion.

"If I owned a monolithic-clad house I'd get it checked, whether I thought it was a problem or not. Experts are saying that 80 per cent of those houses, irrespective of design, will fail."

Ms Cosgriff said most affected home owners were already out of time. The Government had imposed a 10-year time limit on making a claim, so people with houses built in the 1990s had already missed out.

In another three years it would probably be too late, as big councils had refused to sign off on many potentially leaky homes from 2003 to avoid liability.

She believed the risk affected entire suburbs which had sprouted during the building boom, from Dannemora on the southeastern fringe of Auckland to Oteha Valley on the North Shore. "You can go out to Oteha Valley and you drive around and your heart sinks."

Reading between the lines of the latest official report on leaky homes, Weathertightness - Estimating the Cost, a similar picture emerges.

The headline figure, which the report says was devised by Government officials unhappy with the expert prediction, was an $11 billion bill to fix 42,000 homes.

This became the starting point for the Government's failed negotiations with big-city mayors. It was also the official conclusion of the report, which was finished in July but not publicly released until three days before Christmas, a traditional dumping time for bad news.

But the experts' conclusions in the report are based on failure rates of 80-100 per cent for houses with monolithic cladding and untreated timber within 15 years, producing a $23 billion bill for 89,000 houses.

They warn that recorded failure rates do not reflect the real scope of the problem as many people either do not realise they have a leaky home, are in denial or cannot afford to fix it.

They also warn that many homes have been fixed badly and will have to be repaired again and that even drier parts of the country may eventually be affected.

"Some of the buildings from 1995 on are just now starting to show damage in the South Island. The speculation is that the climate has simply reduced the pace of deterioration but that the same fundamental damage is occurring."

Ms Cosgriff said the experts' estimate of a 22 per cent failure rate of all homes was broadly in line with an Auckland City Council estimate of 25 per cent based on claims up to 2008.

She believed it probably underestimated the total damage, as it did not include similar leaks in commercial buildings and only considered the potential liability for the Government and councils over 10 to 15 years.

She dismissed the attempt by Government officials to cut the numbers in half by applying the experts' 80-100 per cent failure rate only to homes which were rated as high risk under the building code's acceptable solution for weathertightness.

Ms Cosgriff said the risk profile used in that calculation assumed house frames would be made of treated timber, which was not the case from 1995 to 2005. Without treated timber, even supposedly safe designs became higher risk.

Home Owners and Buyers Association president John Gray, who was a member of the report's expert advisory group, also rejected the revised figure.

He said many houses with simple designs were leaking too and untreated timber was the real underlying problem.

"I have a sense that every house built from untreated timber is going to be at risk because the assumption is that every house will leak at some time in its life."

He also rejected the report's assumption that the problem had virtually disappeared since the building reforms of 2005, which re-introduced treated timber and forced builders to leave gaps between walls and timber frames so water could drain away.

"There are still houses [that have] been built post-2004 that leak like a sieve. You've still got builders out there who aren't builders, designers who aren't real designers, developers who don't give a shit and some owners who are trying to cut costs and do it as cheaply as they can."

Another member of the experts group, Institute of Building Surveyors representative Russell Cooney, said many home owners missed the 10-year time limit for making claims because there was nothing to show their houses were rotting inside.

"It's not showing through in the wall linings, the paint finishes, the skirting boards and that sort of thing - coupled with the fact that the owners don't know what to look for.

"Many of our members have been to houses where there is not a single sign inside the building of a problem and yet when you put a hole in it you can put your hand through the studs."

The report said home owners would save $1.2 billion if the Government extended the time limit for legal liability from 10 years to 15 years after construction or renovation. This cost would fall on councils, the Government and other defendants.

Building and Construction Minister Maurice Williamson said no decisions had been made but he felt there was a compelling case for more flexibility. He had heard of one man who owned a leaky home for only nine years but was unable to claim because it had been a show home for a year previously.

However, Mr Gray described any time limit as a perverse incentive which should be scrapped.

"I have had a builder bald-faced tell me all he cares about is that the house lasts 10 years and one day. It's totally inconsistent with the intended 50-year life of the house."