"We want to stand with our friends and our allies and our partners. That means we need to be able to back up our commitments and our words - we've got to make sure we are carrying out our piece of it - if that means looking at defence spending, so be it."
The OECD's measurement of defence spending has New Zealand's spend at about 1.5 per cent of GDP. Lifting spending by 0.5 percentage points would mean an increased spend of about $1.7 billion.
Yesterday Act also challenged the Government to lift defence spending, drawing a searing rebuke from Prime Minister Jacinda Ardern that this could lead to cuts in public services, when combined with Act's tax cut pledge.
Luxon conceded this might not be possible, given current constraints on public spending.
National has promised to adjust tax brackets, costing $1.7b if implemented this year -more if implemented after the next election - plus the removal of the top tax bracket, costing $600m, and the reinstatement of interest deductibility, costing $350m-$490m.
The party is looking to fund those changes in its first term in government, when each new Budget is meant to have $3b of new operating spending.
When asked whether National could afford to increase spending levels, Luxon said that was "the conversation".
"I'm not sure we can - the point and the principle is one we're quite interested in.
"We have to go through budgets and work out how we can do it, and how we can manage it - obviously it's a challenging time economically for New Zealand."
Luxon criticised the Government's decision to review defence spending, potentially leading to spending being pushed into the future.
"I don't think the message the New Zealand Government has given recently that they want defence cuts at this time is the right time to be doing anything like that."