What exactly is the "principle" you invoke? If it be that any affiliation with a "gang", however historic, results in an absolute ban on performing a role in prison programmes, then I would submit that the principle is misplaced. Indeed, I would suggest that in the case of carefully selected individuals, the opposite conclusion should be reached.
Those who know the gang subculture from the inside (from when they were young) would be far better placed to make constructive pro-social connections with inmates than persons with good intentions but no life experiences of the circumstances of gang sub-cultures and social deprivation.
On the question of evidence, I also have a broader concern. It is often alleged that Maori are hugely over-represented in the prison population, and that gang members are especially over-represented in the prison population. Numerous reports on the over-representation of Maori in prisons compare the proportion of Maori in prisons with the proportion of Maori in the total population. That is a less than useful comparison.
Criminological research literature from around the world consistently finds prison populations are almost exclusively drawn from young males in the poorest sections of the community with the greatest levels family dysfunction.
Those socio-economic circumstances are also tightly linked to recruitment of young males into groups that provide some support for them but also engage in criminal activity of the sort the police forces focus their attention on. Do you have evidence that Maori gang members are indeed a disproportionate segment of the population when compared with the total of socio-economically deprived young males in the national population - rather than as compared with the total population as a whole?
If your focus and that of the Sensible Sentencing Trust is to impose a blanket ban on all persons who have "gang" affiliations, what is the "evidence" that this will reduce the increasing rates of incarceration in New Zealand prisons?
If your focus is on undermining the appeal of gangs for vulnerable young men who may well commit crimes, then surely government policies should be directed to poverty reduction and community development.
Properly implemented, co-ordinated policies along the lines of Whanau Ora programmes should have the benefit of promoting wellbeing in our poorest communities - including those of many Maori young men who suffer the effects of child poverty and disempowerment but do not commit crimes.
Meanwhile, I submit, the Corrections Department should spend time and effort in recruiting more persons such as Ngapari Nui to be kaiwhakamana in the prisons, rather than standing them down.