Police officers covertly listened to a phone call between a lawyer and a client who had been detained without charge under a preventative detention order (PDO) during September’s counter-terrorism raids in Sydney.

Surveillance of calls is mandatory under the PDO legislation, but the orders had never previously been used since their introduction in 2005, and there is no mechanism for lawyers to be alerted to the fact that clients have been placed on a PDO.

Legal critics have described the covert surveillance as a serious breach of the client’s legal rights.

In September, police raided numerous homes across New South Wales, involving more than 800 officers. Fifteen people were detained amid media reports that the raids had “thwarted a beheading plot”, although no evidence to support this has been publicly revealed. Eventually, one man was charged with a terrorism offence.

The raids were the first time PDOs had been used anywhere in Australia. They allow people to be detained for up to two weeks without charge. Three men were placed on the orders during the raids, but were later released without charge.

Guardian Australia can now reveal what happened to the three men when the orders were issued, although their identities cannot be reported because of a broad suppression order.

Nick Hanna, who represented one of the men when the orders were issued, told Guardian Australia the men were taken to a police station in Surry Hills, where they were held in a separate part of the building from other detainees.

Hanna had a phone conversation with one of the men shortly after he was detained. But when he spoke to his client a second time later that day, he learnt that the first phone conversation had been monitored by police.

In normal circumstances it is highly unusual for lawyers’ communications with their clients to be monitored by police because the confidentiality of the relationship is considered vital to protect the legal rights of a person accused of a crime.

“I called and I had a conversation with my client,” Hanna said. “I then called back about half an hour later and an officer said, ‘By the way, we’re listening to your conversations, we’re monitoring them live and you’re not allowed to speak in any language other than English.’

“I was only told the second time. I hadn’t dealt with this before because they had been so sparingly used, so that was disconcerting.”

Under NSW terrorism laws, all communications from people while they are held under PDOs must be monitored. Any information gained by police from these phone calls cannot be used as evidence against the man.

“The fact that it’s not admissible in proceedings does little to allay our concerns, because there are fundamental reasons for having legal professional privilege that extends far beyond what can be admitted in court, and this really undermines that,” Hanna said.

“At the very least they should be required to notify the lawyer and client that their communications are going to be monitored.”

Hanna said the police had still refused to provide reasons for holding his client, who has not been charged.

“Shortly after he was released we put in a formal request for the reasons to be provided for us,” he said. “All we were ever told is that it is terrorism-related, yet nothing’s been produced. We’re not aware of anything adverse being found from when his house was searched.”

Hanna said his client had sustained injuries when he was detained, and he was considering whether to take action against the police.

Guardian Australia is unable to identify Hanna’s client because of a broad and indefinite suppression order issued by a NSW supreme court judge.

NSW Police sought and were granted 36-hour PDOs by the court the night before the raids. The order said the name of the parties, the evidence and the judgement could not be published. It is still in force and appears to apply indefinitely.

PDOs were introduced by Commonwealth and state governments after the 2005 London bombing, and were designed as a response to an imminent terrorist attack. However, the federal government’s former national security legislation monitor has described them as an “unnecessary” addition to existing police powers.

The PDO regime is so restrictive that the Australian Federal Police even refused to initially confirm how many people were being held shortly after the raids.

Hanna’s account of the use of the law has raised concerns with legal experts. Australian Lawyers Alliance spokesman Greg Barns said it was a serious breach of the client’s legal rights.

“It’s outrageous that security and police agencies are listening in on the conversation between clients and their lawyers,” he said. “It breaches a fundamental human right which is the right between a lawyer and their client, and it’s a breach of the rules relating to legal professional privilege.”

Guardian Australia put detailed questions about the circumstances of the use of the PDOs to NSW police. A spokesman said in a statement: “It is inappropriate for NSW police to comment.”

PDOs can be issued under NSW or Commonwealth laws. Laws allowing the federal police to seek the orders were due to expire in 2016, but have been extended by parliament as part of its second tranche of national security legislation.

Omarjan Azari, the only person charged with a terrorism offence following the raids, appeared in court in November. His lawyer told the court there had been a “glaring error” in the translation of an intercepted phone call at the centre of the case.