Christopher Luxon continues to dismiss claims the numbers in National's tax cuts plan don't stack up, saying "Nicola Willis and I are good with numbers".
Last week, after National unveiled its $14.6 billion tax relief plan, Labour finance spokesperson Grant Robertson immediately went on the attack - saying the plan "didn't add up and will make life harder for Kiwis".
Robertson said there were aspects of National's tax plan falling apart "every day".
"National is relying on bringing in nearly $3 billion from foreign buyers to fund their tax cuts, but nearly every expert says those numbers simply aren't possible," he said on Sunday. "If National's revenue streams don't add up their tax cuts are unaffordable."
But, speaking to AM on Wednesday, Luxon said the plan was "rock solid".
"It absolutely stacks up and you should know that [National finance spokesperson] Nicola Willis and I are good with numbers and good on economics, and it's in good shape," he told co-host Laura Tupou.
"The important thing here is, what we're doing is we're raising revenue, reprioritising spending and giving money back to hard-working Kiwis - what we call the 'squeezed middle.'"
Luxon was confident the revenue could be raised from the streams it was proposing, which included partially lifting the ban on foreign buyers while slapping them with a tax, as well as offshore online gambling and user-pays immigration levies.
"We're very confident we can," he said.
"We've spoken to a number of experts around free trade agreements, around tax treaties, around online gambling and we are very, very confident we can do that.
"I'll tell you; our modelling says that it can happen - we've been very conservative, actually, with our numbers."
Luxon added the proposed 15 percent foreign buyers' tax on homes over $2 million would go "straight to lower-middle income earners to give them a break".
"It's not 'suppose' - it is going to happen," he explained to Tupou.
Luxon's comments come after Newshub revealed on Tuesday it would actually be illegal to license new online gambling operations and required a law change.
On Sunday night, it was revealed National only sought independent advice on whether its foreign buyers' tax could break international law two days after the policy was released.
Despite this, Luxon - the person hoping to be New Zealand's next Prime Minister - said the experts National had consulted with were "great".
"We're very confident all of that is completely doable."
When asked by AM where the supposed foreign buyers would come from, Luxon said "all over the world".
Partially lifting the ban on foreign buyers - although slapping them with a tax - would bring wealthy foreigners into New Zealand and have a positive effect on the economy, he believed.
"These are people that can afford $2 million-plus houses and are going to pay a 15 percent tax... there's talent and capital and money that people want to invest in New Zealand."