About 15 to 20% of New Zealand prisoners are in maximum security. The rest of these prisoners are in medium to low security prisons where it is much easier to escape.
Most New Zealand prisoners have about 40 to 50 previous convictions, with serious and violent assaults (21%), sexual offences (20%), home invasions and burglary (14%), aggravated robbery and robbery (9%), and homicide (7%) which together add to 71%. Drug traffickers make up another 12%.
My point is most New Zealand prisoners are serious offenders but most of them can be trusted not to escape even when in low security and prison farms. The threat of returning to maximum security upon recapture is incentive enough to keep them on the straight and narrow.
There is quite a serious literature on how variations in prison conditions, prison overcrowding and deaths of prisoners acts as a deterrent. You do not have to watch all that many American TV shows to notice they plea-bargain with promises of a prison near their family, in a warmer climate and lower security rating. Maximum-security, far away and surrounded by gang members is more than enough to keep most prisoners in line.
The strongest argument that prison deters crime is made by opponents of 3 strikes legislation. They claim that without the prospect of parole, prisoners are be more difficult to manage in prison. That is an incentive argument, that the dim prospect that parole perhaps decades hence has powerful incentive effects. QED