The head of the police says its list of known gang members is not a statistical measure of how many there presently are operating and shouldn't be used that way.
The National Party earlier this week used it to claim gang membership in New Zealand is exploding, up 46 percent since Labour took power from them in 2017.
"We had it relatively stable," National leader Judith Collins told The AM Show on Wednesday. "What we're seeing now is almost a 50 percent increase in just three-and-a-bit years."
There were 5343 people on the list in 2017, compared to 7825 now.
Police Commissioner Andrew Coster told The AM Show on Friday there has been a concerning rise in "violence between gangs", but it was hard to say whether gang numbers had actually gone up.
"The gang membership question is difficult. It's very easy to get on the national gang list, it's much harder for us to know when people should be taken off it. It probably is only going to go in one direction."
Coster's views align with those of Jarrod Gilbert, director of criminal justice at the University of Canterbury and author of Patched: The history of gangs in New Zealand.
"Without question, those numbers are really heavily inflated so when we hear things like, 'the gang numbers have doubled in Wellington in the last three years,' it's simply not true," he told The AM Show on Thursday.
Gang patches are easier to earn than they used to be, he said.
"What that means is easy come, easy go. So you're seeing a lot of people join, realise it's not for them for whatever reason, then dropping off."
Coster said the list was created "for intelligence purposes".
"It's obviously really important for our people to know about those associations. It wasn't designed to be a statistical count, although it is being used that way by some."
If it was a statistical count, the rise in numbers is nothing new - the list's membership rose more than 10 percent a year between 2014 and 2017, when National was in power - from 3960 in mid-2014 to 5343 in 2017.
"The difficulty is to know when someone might have drifted away from gang activity," said Coster.
"So if someone tells us they've left the gang, that's probably not that credible. We want to look for independent evidence of that, but we don't always get it.
"The bottom line is belonging to a gang is not a criminal offence - we have to focus on the organised criminal offending that is connected with gangs, and we are doing that very successfully."
National's Simon Bridges has targeted Coster as a "wokester", criticising his record on tackling gangs. Coster chuckled when asked about the term, saying he'd rather "focus on the issues".
"I welcome the robust debate, and you get that with the big jobs. It's all part of being a democracy."
He said 2020, despite the COVID-19 disruption, saw the police confiscate more firearms and lay more charges against gang members than ever before.
"The arguments here need to be based in fact - and the fact is we are having a massive impact."