National says prisoner numbers will rise as crime goes down if it wins power in October.

The party's new crime policy would make being a gang member an aggravating factor during criminal sentencing.

Leader Chris Luxon told Mike Hosking that there'll be more to come in its broader policy to be announced shortly – and more prisoners will be a consequence of chasing down crime.

He says the Government has emptied out prisons by a quarter in the past six years, but crime is up 30 percent, and retail has jumped 40 percent.

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