Australia has ratified the Optional Protocol to the Convention against Torture – eight years after signing the treaty – acceding to independent inspections of all places of detention in Australia.

The government will formally lodge documents to ratify the Opcat, which builds on the 1985 UN Convention against Torture, in New York next week.

Under the Opcat, Australia will be obliged to establish an independent watchdog, known as a national preventive mechanism, which will have standing powers to conduct regular and random inspections of prisons, immigration detention centres, juvenile detention centres and held psychiatric facilities.

Australian places of detention will also be subject to international inspections by the UN subcommittee on the prevention of torture.

But the watchdog will not have the right to inspect offshore places of detention run by Australia.

The government committed to ratifying Opcat as part of its successful campaign for a seat on the UN’s human rights council.

In a statement on Friday, the attorney general, George Brandis, and foreign minister, Julie Bishop, said the government was committed to ensuring implementation was practical and effective.

“Ratification is not the end, but the beginning of an ongoing discussion about oversight and monitoring,” it said. “Ratifying Opcat demonstrates Australia’s unwavering commitment to international scrutiny and accountability, as Australia prepares to take its seat on the Human Rights Council for the 2018–20 term.”

There is a significant difference between signing an international treaty and ratifying it. By ratifying a treaty, a state consents to be legally bound to adhere to it.

Multiple submissions to the Northern Territory royal commission into the protection and detention of children called for Australia to ratify the Opcat, to strengthen inadequate monitoring processes.

The royal commission found systemic failings of the child protection and justice system in which juveniles were mistreated and, in some instances, abused over the course of 10 years.

Similar concerns, including the alleged use of excessive force, subsequently emerged about Tasmanian, Victorian, and Queensland facilities.

Had the Opcat been in place, the NT centres would have been subject to random unannounced compliance checks, which could also have identified structural issues such as the existence of multiple hanging points.

The royal commission’s final report, delivered last month, found “shocking and systemic failures” in a system which “failed to comply with the basic binding human rights standards in the treatment of children and young people”.

“Ratification of OPCAT and the introduction of a system of inspection mechanisms will directly address concerns raised by the commission about the lack of accountability and transparency in youth detention facilities in the Northern Territory,” the report said.

The Australian human rights commissioner, Ed Santow, told Guardian Australia in February the Opcat would not set up new complaint mechanisms but would make detention reviews more systematic.

“Take a hypothetical example like Don Dale – if Opcat was in place then an inspector would have gone into Don Dale and asked some serious questions about why spit hoods were used on juvenile detainees,” he said.

The Human Rights Law Centre said ratification of Opcat was an important step towards transparency because successive Australian governments had maintained an “out of sight, out of mind” approach to places of detention for too long.

But Daniel Webb, the HRLC’s director of legal advocacy, said the Australian government could not just pick and choose which facilities it was happy to have scrutinised, and to shield offshore detention from inspection.

“The whole purpose of this treaty is to prevent abuse through transparency. It defeats that purpose if our government can just decide to keep its deepest darkest sites of misery and suffering deliberately shielded from scrutiny,” Webb said.

Webb said the timing of the ratification — just before Australia begins its term on the Human Rights Council — showed that the government recognised the need to improve its human rights performance before taking a seat on the world’s highest human rights authority.

Save the Children director Mat Tinkler said the ratification of the Opcat had been a recommendation of several inquiries.

“We must have strong, independent safeguards and oversight around conditions in places of detention, and the ratification of Opcat goes a long way towards achieving this,” he said.

“However, we remain concerned that this announcement, whilst welcome, is unlikely to have any direct benefit to those children and their families still held within Australian run offshore immigration facilities such as those on Nauru.”