Four inmates were observed by prison officers absconding from a Darwin jail, but were not pursued by staff because to "chase prisoners is not in our protocols", a corrections authority said.

Key points: Inmates thought they could leave and re-enter undetected

Inmates thought they could leave and re-enter undetected Prisoners not considered a threat to public

Prisoners not considered a threat to public Inmates were later moved into higher security area

The minimum security prisoners, Christopher Keene, Ian Mamarika, Jayras Manggurra and Xavier Nundhirriballa, were seen climbing a fence near low security "pre-release" accommodation units shortly after the evening roll call at 7.00pm at the Holtze facility, near Darwin, on Saturday.

The prisoners were found to be absent at a roll call after they were "seen close to the boundary fence that surrounds the cottage complex".

Executive director of custodial operations at the prison, Bill Carroll, said security did not go after the prisoners when they were seen climbing the fence.

"It was dark and the prisoners can hop into the bush pretty quick, there are only two staff down here and to leave the immediate area and chase prisoners is not in our protocols, so they did the right thing," he said.

An alert was issued on social media on Saturday night which assured the public the prisoners were not regarded as a threat.

Prisoners at the low security corrections facility at Holtze, in Darwin. ( ABC News: Avani Dias )

"It's pretty evident the prisoners didn't intend to escape long-term. They obviously went into the bushes and were mischievous with the intention to come back," Mr Carroll said.

"Normally when four prisoners will go it's not orchestrated, it was probably opportunistic."

The prisoners were seen trying to re-enter the facility shortly after 1:00am Sunday.

"The four prisoners are now in a high security environment and they'll be there while an investigation is conducted," Mr Carroll said.

He said the group was facing charges of escaping and could spend up to three months in the high security facility.

Mr Carroll said they would then be sentenced by a court and spend extra time in jail.

"They certainly won't be invited back into this [open security] program."

The four prisoners were rated as "open security", allowing them to leave the prison complex during weekday working hours to engage in employment or voluntary work in the community.

Prisoners in the open security facility have committed what are considered "low-level" crimes.

"It's an open environment, this environment is designed to be as close as it can to represent normal life in the community," Mr Carroll said.

"The prisoners aren't secured into their own cells, in fact they have their own cell keys so they can secure their rooms when they go to work.

"They could walk out the front door but that's not the part of the program but there's quite a strict criteria that the prisoners have to meet to ... be placed in the position."

Break-outs, break-ins plague NT jails