Beijing has charged an Australian journalist who worked for Chinese state television with illegally supplying state secrets overseas, six months after she was detained.

The Australian government confirmed it had received a formal notice of Cheng Lei’s arrest on Monday, long after Chinese authorities took the Australian national into China’s opaque system of secret detention centres.

The formal notice of Cheng’s arrest comes as Australia-China relations have sunk to their lowest level in a generation, following Canberra’s call for an international probe into the origins of Covid-19 and its introduction of tough laws against foreign interference.

Cheng was a newsreader for China’s state-owned China Global Television Network. She is the second high-profile Australian to be detained by Beijing in recent years, after writer Yang Hengjun was also held and charged with espionage in 2019.

“Chinese authorities have advised that Ms Cheng was arrested on suspicion of illegally supplying state secrets overseas,” said Marise Payne, Australia’s foreign affairs minister.

The Australian government said embassy officials had visited Cheng six times since her detention. A six-month detention is common for foreign prisoners held on national security charges.

Foreign governments have become concerned that Beijing is increasingly willing to hold their nationals as bargaining chips.

Michael Spavor and Michael Kovrig, two Canadians, were detained shortly after Huawei chief financial officer Meng Wanzhou was arrested in Vancouver following a US extradition request in 2018. They were also held under the country’s “residential surveillance at a designated location”, or secret prisons programme, for six months.

Beijing’s use of such detentions as a political tool has accelerated under President Xi Jinping. Almost 30,000 people have been held in the facilities from 2013-19, according to Safeguard Defenders, a human rights group co-founded by Swedish citizen Peter Dahlin, who was himself held in a secret prison.

Louisa Wen, who is Cheng’s niece and is acting as spokeswoman for the family, told Australian media that the conditions under which her aunt was being held had deteriorated recently and her children were devastated by her absence.

“We don't understand anything about the case . . . But we do know she's been in detention for five-and-a-half months, and her conditions are worsening,” she told Australia’s state broadcaster ABC.

Wen added: “We don't know if she's just been caught up in something that she herself didn't realise.”

Richard McGregor, an analyst at the Lowy Institute think-tank in Sydney, said the formal arrest of Cheng would be devastating for her family but was not unexpected, given she has been held in detention for six months — a legal limit under Chinese law.

“What is still not clear, I suspect to anyone but the Chinese government, is what state secrets she is alleged to have disclosed. In China, almost anything can be classified as a state secret, which makes it an offence that the state can do what it likes with,” said McGregor.

China’s legal definition of state secrets is extremely broad, covering topics related to the country’s economic and technological development as well as military and national defence.