Today, he will show the media some areas of Auckland where he says young couples could buy homes using the beefed-up grants.
Mr Key said the Treasury and the Reserve Bank were aware of the promises National was making.
Last year, the Reserve Bank imposed rules on banks to increase the deposit required on loans in a bid to address fast-rising house prices.
"This is a very powerful policy for first-home buyers," Mr Key said.
In essence, the Government was providing a much bigger deposit for a newly built home, which could be used for a Crown-guaranteed loan under the Welcome Home scheme.
As an example, a couple in Auckland each earning $50,000 and who have been in KiwiSaver for five years will be able to withdraw $35,000 and receive a $20,000 Homestart grant for a new-build home, giving them a $55,000 deposit.
What do you think of the HomeStart policy? Send your thoughts to newsdesk@nzherald.co.nz
Because the Welcome Home Loan requires only a 10 per cent deposit, they will be able to buy a home priced at up to $550,000.
The Welcome Home scheme is exempt from the Reserve Bank loan-to-value rules.
Mr Key said National valued home ownership because it provided stability for families, strength for communities and security in retirement.
"We want to help people into their first home, but they have to help themselves first."
Mr Key and Housing Minister Nick Smith estimated that over the next five years, 90,000 grants would be made under Homestart and of those, 40,000 would be because of the new policy. About 22,500 would be newly built houses.
The estimated cost of Homestart for five years is $435 million, $217 million of which is for the new policy.
Auckland is projected to grow by a million people over the next 30 years, and the Auckland Council in December 2012 identified a shortfall of about 20,000 to 30,000 new dwellings and a need for 13,000 new homes each year for the next 30 years.
The Auckland Housing Accord, agreed to last year by the council and the Government, has created Special Housing Areas and fast-tracks building consents for homes in both inner-city and city-fringe areas.
Sixty-three areas and 33,500 homes have so far been announced, with a target of 39,000 properties.
To qualify, a couple must earn less than $120,000 and singles less than $80,000. Buyers would have to have been in KiwiSaver for at least three years and plan to live in the house for at least six months.
The maximum purchase price would be $550,000 in Auckland, $450,000 in Hamilton, Western Bay of Plenty, Kapiti, Porirua, Hutt Valley, Wellington, Nelson and Christchurch and $350,000 elsewhere.
But commentator Bernard Hickey said the policy would do nothing to solve the shortage in Auckland, where the council estimates a need for 13,000 new homes a year for the next three decades.
Read a summary of the HomeStart changes
Plan offers hope for young couple
Aucklander Aya McKenzie says National's first-home package could help her on to the property ladder.
The 27-year-old digital technician and husband Nathan, a chef aged 28, have been looking for a home on Waiheke Island. They have a deposit of more than 10 per cent for a home in the $500,000 to $600,000 bracket.
Ms McKenzie is not a member of KiwiSaver, but her husband is and qualifies for an extra grant if they decide to build a new home. The couple have seen an 800sq m section for sale at $180,000 and believe they could build a house for $250,000.
Chuang turns up to show support for Team Key
Len Brown's former mistress showed support for Team Key at the National Party's election campaign launch in Auckland yesterday. Bevan Chuang, who had a two-year affair with the Super City mayor, attended the launch at the Vodafone Events Centre in Manukau.
Ms Chuang tweeted throughout the function, making full use out of National's election hashtags #TeamKey #Working4NZ.
Bevan Chuang.
This message linked to a photo of Mr Key at the event on Ms Chuang's Instagram account:
"Prime Minister Hon John Key #TeamKey #Working4NZ @ Vodafone Events Centre."
Tame Iti seeks election under Maori Party flag
Tame Iti will stand as a list candidate for the Maori Party in this year's election. The Tuhoe artist and political activist said the Maori Party was the "only independent Maori voice in Parliament. The Maori Party has got credibility. They've established themselves as an independent Maori movement that walks the talk. I also support them sitting at the decision-making table of Government; it's the only way Maori can bring about change. It's no good moaning on the outside."
