North Australian Aboriginal Justice Agency (NAAJA) CEO Priscilla Collins says the NT government's focus on a tough-on-crime approach has sent the imprisonment rate soaring.
"You have so many people overcrowded in there, something's going to happen," she told AAP.
The Country Liberal Party-led government is building a new 1000-bed prison at a cost of half a billion dollars, due to open next year.
But critics say that with current rates of imprisonment, the new jail will be more than 80 beds short when it opens.
Ms Collins says 85 per cent of prisoners at Darwin's jail are Indigenous.
She's criticised the NT government for locking people up rather than treating the underlying causes of crime.
"You're letting them back on the street and you haven't dealt with the problem," she said.
"You've just separated them from their family, they don't have an income, they'll have to start from scratch to reintegrate in the community, when all you had to do when they first came into contact with the justice system was have more therapeutic programs available so they can deal with those issues."
The Australian Greens have advocated justice reinvestment programs, which redirect funds into community-based programs to limit offending.
Every year Australia spends $3 billion on running jails and billions more on courts and policing, says Senator Penny Wright, the Greens spokeswoman on legal affairs.
"In the last 30 years the prison population in Australia has tripled, yet people don't feel any safer," she told reporters in Darwin on Thursday.
The Greens are calling on the federal government to take leadership on the issue of justice investment programs, and to encourage the states and territories to do the same.
