ACT is challenging New Zealand First MP Ron Mark to use his party's cosy relationship with Labour to pressure the government over the burgeoning prison population.

Deputy leader Heather Roy says the Corrections Department is battling record numbers of inmates.

She says Ron Mark is suggesting that prisoners be sent to Waiouru or other military bases.

Mrs Roy says that is not the solution and Mr Mark needs to convince his Labour colleagues to bring back privately-run prisons like the Auckland Central Remand Prison.