Police in the Northern Territory have the power to hold a person in custody for up to four hours without a warrant if they reasonably believe they committed, were committing or were about to commit one of 35 different offences, in a practice known as a "paperless arrest". Most of the offences are minor, including swearing and failing to keep a front yard clean.

Under the law, the person is then either released on bail with an infringement notice or brought before a court for the alleged offence.

If the person is intoxicated, they can be held until the police reasonably believe they are no longer intoxicated.

At least one person has died in custody following a paperless arrest.

Legal service the North Australian Aboriginal Justice Agency and an Northern Territory coroner have raised concerns that police disproportionately arrested Aboriginal people using the powers.

Documents obtained through freedom of information have shown that 525 of 731 paperless arrests from January to March were of Indigenous people.

NAAJA and Miranda Bowden - who was held in custody for almost 12 hours under the law - asked the High Court to declare the law invalid. They argued it gave the Northern Territory government judicial powers that were punitive and penal, beyond its constitutional powers and in breach of the separation of powers. It also removed the courts' judicial oversight powers from such detention, they said.

Six of the seven High Court judges found on Tuesday that the law was valid.

Chief Justice Robert French and justices Susan Kiefel and Virginia Bell ruled that the powers were not for a "punitive purpose" but administrative. The powers were limited to circumstances where arrest was "appropriate", including when it's needed to ensure the person was available to be dealt with over the offence, and to prevent an offence being completed or repeated.

They also noted that if the maximum time period a person could be detained were "significantly greater" than the law allowed, it may raise questions on whether it is justified.

Jonathon Hunyor, NAAJA's principal legal officer, said that the High Court had clarified that once an infringement notice was issued, police could not continue to hold such people in custody. They could only release them or take them before a court.

People were typically held in custody for hours after they were issued an infringement notice, he said: "That was the police practice; that was the police policy. We're disappointed with the outcome but we're pleased that the High Court's reined in what's otherwise a bad law."

The decision paves the way for Ms Bowden's case against the Northern Territory to be determined separately, with the High Court referring it back to a single judge for further directions.

Ms Bowden was arrested in Katherine in March, and issued with $274 fine when she was released from custody for using obscene language, indecent behaviour and bringing liquor into a restricted area. She is seeking damages for wrongful imprisonment.

"Now that the court's told us what [the law] means, we have to decide whether it applied in that way in her case," Mr Hunyor said.

Indigenous artist Kumanjayi Langdon, 59, was issued with an on-the-spot fine in May on suspicion of drinking alcohol in a Darwin park. He was taken into custody and found dead about two hours later, having suffered heart failure linked to alcohol consumption.

Coroner Greg Cavanagh recommended that the law around paperless arrests be repealed, saying: "In my view, unless the paperless arrest laws are struck from the statute books, more and more disadvantaged Aboriginal people are at risk of dying in custody, and unnecessarily so."

NT Attorney-General John Elferink has defended the legislation, saying it allowed a "truncated police procedure", saving officers hours of paperwork for minor offences that would "very likely lead to a guilty plea."

Justices Geoffrey Nettle and Michelle Gordon said that it was unlikely that police could legitimately arrest someone rather than issue them an "on the spot" fine for very minor offences "unless the offence is continuing or there is an ongoing risk to public safety or order." When police powers were abused, there were "well-established" remedies, including civil actions against police where compensation could be awarded.

"And, in the Northern Territory, a deliberate delay in bringing a person who has been arrested before a court is a crime punishable by imprisonment," they said.

Justice Stephen Gageler dissented, ruling that the law was invalid because it was not constrained to ensure the person is only detained as long as it is practicable to allow them to be brought before a court or dealt with in another way.

"Nor is the discretion constrained to be exercised only in a manner which ensures that the detention is protective of the person or of other persons or preventive of harm," he said.

He said the law also impaired the "institutional integrity" of Northern Territory courts contrary to the constitution, because "paperless arrests" ultimately meant that the executive, rather than the judiciary, has detained them.

"No subsequent action by a court can change that historical fact," Justice Gageler said.

"Courts of the Northern Territory are thereby made support players in a scheme the purpose of which is to facilitate punitive executive detention. They are made to stand in the wings during a period when arbitrary executive detention is being played out. They are then ushered onstage to act out the next scene. That role is antithetical to their status as institutions established for the administration of justice."

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