The briefing paper said the prison population was about 8700 people, up from 8500 this time last year.
Two factors were identified as contributing to the larger population -- a small increase in violence and Class A drug offences, and a law change in October last year that made it harder for people facing murder charges or repeat violence, sex or drugs charges to get bail.
Mrs Collins' law change reversed the burden of proof for accused offenders in bail cases, and was expected to lead to 350 more people a year being refused bail.
The National-led Government does not have a specific goal for reducing the prison population. But it wants to reduce reoffending by 25 per cent between 2011 and 2017, which should lead to fewer people being locked up.
The Government has estimated that if it reaches its reoffending target there will be 600 fewer people in prisons than in 2011, meaning the prison population would be around 7950 in 2017.
Mr Lotu-Iiga said the growth in prisoner numbers was not a cause for concern "at this stage" and was unlikely to affect planning or policy.
Prison population
• Sept 2010 - 8,747
• Sept 2011 - 8,488
• Sept 2012 - 8,623
• Sept 2013 - 8,474
• Sept 2014 - 8,700 (estimate)
- Source: Corrections NZ