Will the Govt promise affordable homes? 'Depends on your definition' - Steven Joyce

Labour finance spokesperson Grant Robertson won't rule out a capital gains tax (CGT), should he become Finance Minister after this year's election.

The party went into the last election with a policy of introducing a CGT on properties apart from the family home, but that was ditched when Andrew Little became leader.

Mr Little recently stepped aside so Jacinda Ardern could take over, and now the policy is back on the table.

Appearing alongside his National counterpart Steven Joyce on The Nation on Saturday morning, Mr Robertson said the party is working on a new CGT policy, but it won't be unveiled before the election.

The existing two-year bright-line test, which applies to properties flipped within two years, would be extended to five years under Labour.

Mr Robertson also committed to spending more on Labour's Kiwibuild policy, should it need more to cover growing building costs.

"We're committed to making the project work. If we need to put more money into it, we will.

"One of the great things about a Government coming in and actually building some houses is we can build at scale, and that brings costs down."

The plan is still to kick off the 10-year scheme with $2 billion, which would replenish as the homes are sold. The party will not scale back the plan if building costs balloon further.

"Absolutely not," said Mr Robertson.

"The shortfall in houses has grown significantly under National's watch. Bear in mind, if we had started this proposal in 2012, we would have already built tens of thousands of houses."

He says in the last 10 years, only 5 percent of homes built have been affordable.

Mr Joyce disputes that figure, saying it "depends on your definition" of affordable.

But he refused to make any guarantee affordable homes would be built over the next three years, should National remain in power.

"I'm going to guarantee that first-home buyers will continue to get the opportunity to participate in the New Zealand housing market as they are today."

He said there are presently 100,000 homes built every three years - which is how many Labour plans to build in 10 - but wouldn't say how many of those were affordable.

"They're a range right across the board."