If National takes back power in Parliament next year, party leader Simon Bridges said he will move to block gang members from the benefit who can't prove their income or assets have been obtained legally.
It's part of National's Social Services Discussion Document, which is set to be released in full today at midday.
It comes after Mr Bridges tweeted he's seen the "misery gangs peddle" and if he was prime minister he'd make sure they couldn't "exploit taxpayers".
However, TVNZ1's Breakfast host John Campbell quizzed Mr Bridges this morning on how it would be proved.
"It would be quite simple," Mr Bridges said. "You'd go through what they've got, and they say where they've got the money from."
But when pressed on who they would go through that with, Mr Bridges added that it would come out in the court system if that's where it got to, or for the Ministry of Social Development to work out.
"It's pretty simple, isn't it? If you've got a Harley Davidson motorbike, you should be able to say within reason, 'You know what, I've been working as a chippy for the last 10 years, I've earned this much money and that's how I've paid for it'.
"But here's the thing with gang members - there are thousands of them who have Harley Davidson motorbikes and haven't been working as a chippy for several years. The average gang member - 92 per cent of them, in fact - have been on a benefit for an average of nine years.
"Call me old fashioned, I sit there and say that's not right."
Mr Bridges also said gang numbers had risen by 1400 members in the past couple of years.
"I know there are some views out there that say, 'Oh, you know, it's about whānau and community' - I don't buy that. I think it's about peddling misery, about drugs including methamphetamine and the growing, ever-growing problem we have with that, and violence.
"So either we want to be serious about this - and National does - or we don't and we will continue to see this problem that fuels our drug problem in New Zealand grow."
He said there would be a "huge emphasis on social investment" in the discussion document out today, including in the first 1000 days of a baby's life.
However, Mr Bridges added: "You can't do the one side but say, 'But you know what, where there is serious wrongdoing we'll ignore that'."
He said National will take a "firm but fair" and "balanced" approach to the issue.
Auckland Action Against Poverty has called National's social welfare policy "out of touch", and claimed it would only further hurt low income families.