It is expected that 15,000 to 25,000 buildings will be assessed as earthquake prone - a figure that could rise - and all will have to be strengthened or demolished within 15 years of the legislation taking effect.
At present, owners have about 28 years on average to bring their buildings up to code or demolish, Mr Williamson said.
Opposition leaders are critical of the plan, saying there is not enough information to put forward a realistic strategy.
Labour's building and housing spokesman, Raymond Huo, said the Ministry of Business, Innovation and Employment's consultation document did not contain enough data to be able to accurately gauge how much the changes could cost or the true number of earthquake-prone buildings in New Zealand.
"If the Government's own ministry has insufficient data at present to give a clear indication, it's not good enough," Mr Huo said.
The ministry points the finger at councils, citing "poor information on the number and specific location of earthquake-prone buildings across the country, due to inadequate data collection".
The ministry also points to failings by central government, for providing limited information and guidance to local authorities on stronger buildings.
The plan has also met with outrage from some civic leaders and landlords.
Dunedin Mayor Dave Cull, Otorohanga Mayor Dale Williams and Hastings Mayor Lawrence Yule, who is also president of Local Government NZ, have spoken out against the proposals, claiming provincial towns and rural communities would be financially ruined.
Timaru Mayor Janie Annear has described the proposals as devastating.
But Mr Williamson said the Canterbury Earthquakes Royal Commission report showed the Government had to take the issue seriously.
The minister said assessments would initially be a desk-top exercise, examining materials, design and structure, and then if an owner objected, a more detailed analysis by an engineer would be needed.
"We must ensure the earthquake-prone buildings policy system strikes an acceptable balance between protecting people from serious harm, and managing the significant economic implications of strengthening or removing the most vulnerable buildings," he said.
The Government was mindful of the potential cost put on owners but the destructive Canterbury earthquakes highlighted the need to review and improve the system of dealing with unsafe buildings.
Labour's Mr Huo said he understood why mayors were so worried.
"It's not right for the minister to criticise the local mayors, because they are kept in darkness and of course are concerned about the possible loss of their heritage. If you talk to building owners in Christchurch, not many can afford to get their buildings strengthened to 33 per cent and insurance companies refuse to cover the costs associated with strengthening issues," Mr Huo said.
"Of course building owners, consumers and local mayors are concerned and are going to the minister for a solution. But when they do this, they're being told it's their problem," he said.
Mr Williamson has suggested building owners could be required to do as little as posting a sign saying the building was not up to earthquake strength.
But Mr Huo said this was plainly inadequate.
"I think that is a pretty flippant option ... It's a hands-off approach, it's somebody else's problem."
He had talked to builders, engineers, designers and subcontractors and said the feedback was that the Government needed to balance various interests, including public safety, costs, heritage, the viability of businesses and the wider interests of small communities.
Asked what alternative Labour proposed to the Government's scheme, Mr Huo said its policy had not yet been formulated, nor had Labour released an official statement in response to Mr Williamson's stance or the ministry's consultation document.
Mr Williamson said the proposals were up for public comment. A consultation document was issued after the Canterbury Earthquakes Royal Commission report was released to the Government in October and to the public on December 7.
Meetings have been held in the past few weeks to get feedback about the issues and submissions on the document close this Friday.
Mr Williamson said the Government would release its decision around the middle of this year.
Q&A: Earthquake upgrade
What is the Government plan?
* Assess all non-residential buildings and multi-unit, multi-storey residential buildings within five years.
* Buildings judged to be quake-prone will need to be strengthened or demolished within 15 years.
* That allows five years for local authorities to complete assessments, followed by 10 years for owners to strengthen or demolish.
* 15,000 to 25,000 will need to be strengthened or demolished based on current estimates, but this figure could climb.
* Pre-1976 buildings will be the focus.
What is a quake-prone building?
* A building that is not built to 33 per cent of the standard required of a new building.
* A building with unreinforced masonry.
What buildings are exempt from the plan?
* Houses.
* Woolsheds, farm sheds and cow sheds.
* Small rural community halls.
* Rural churches.
* Seldom-used buildings.
Why is the Government doing this?
* The proposals are a response to the recommendations of the Canterbury Earthquakes Royal Commission.
What can building owners do?
* View the consultation document
* Submissions close on Friday.
What happens next?
* After the consultation period finishes, the Ministry of Business, Innovation & Employment will analyse feedback and submissions and report to the Government for it to make decisions. A decision is expected in the middle of next year.
