China ADMITS why it arrested Australian journalist after two reporters were forced to flee the country 'over fears for their safety'

Chinese authorities have revealed why an Australian journalist has been detained in Beijing after two other reporters were forced to flee the country.

Cheng Lei, a high profile business anchor on Chinese state television, has been held in a secret location for more than three weeks. 

'The Australian national Cheng Lei is suspected of carrying out criminal activities endangering China's national security,' Chinese foreign ministry spokesman Zhao Lijian told reporters.

Australian journalist Cheng Lei was detained in China's capital city Beijing in August. She is a high profile anchor for CGTN, China's English language state broadcaster

The Chinese foreign ministry said Ms Cheng had been investigated and was 'suspected of carrying out criminal activities endangering China's national security'

'Compulsory measures have been imposed on Cheng and she has recently been investigated by relevant authorities.'

He said the case was being handled according to Chinese law and Ms Cheng's legitimate rights and interests were fully guaranteed.

The explanation comes after two Australian journalists were rushed out of China for their own safety after being banned from leaving until they answered questions about Ms Cheng.

Bill Birtles from the ABC and Michael Smith from the Australian Financial Review returned to Sydney on Tuesday after a five-day diplomatic stand-off.

Their rushed evacuation has left Australia without any correspondents working on the ground in China for the first time in 50 years.

Chinese police told the journalists they were people of interest in the case of Ms Cheng and both men were ordered to report for questioning.

They sheltered in Australian diplomatic compounds for days as their travel rights were revoked.

It is not known where in Beijing Cheng has been detained since August

The explanation for Ms Cheng's arrest comes after two Australian reporters, Bill Birtles from the ABC and Michael Smith from the AFR, fled China fearing for their safety 

Consular officials secured safe passage after the pair agreed to be interviewed.

Both journalists say they are relieved to be home but disappointed about the circumstances surrounding their departure.

The Department of Foreign Affairs urged the ABC to withdraw Mr Birtles from China last week after it learned of Cheng's detention.

Foreign Minister Marise Payne said the government had become concerned about the uncertainty posed for other Australian journalists.

Mr Birtles and Mr Smith were initially banned from leaving the country until they answered questions about Cheng. Both reporters had been told they were persons of interest 

Mr Smith holds his thumbs up upon his return to Sydney on Tuesday morning. He and Mr Birtles were questioned by Chinese authorities before they could return to Australia

Mr Birtles is seen at Sydney airport on Tuesday following the frightening rush to leave China

Mr Zhao told reporters the questioning of the men was a normal enforcement of the law, which authorities had strictly adhered to during their investigations.

He said China protected the legitimate rights and interests of news gathering staff and they had the obligation to comply with the laws and regulations in China.

'As long as foreign journalists conduct news reporting in accordance with laws, they should have nothing to worry about,' Mr Zhao said.

The Australian government is advising all Australians not to travel to China, warning they could face arbitrary detention.

Australia has been left without any credentialed journalists in China for the first time since the 1970s.

Senator Payne described the diplomatic saga as a disappointing.

'Australia, of course, is a strong advocate of freedom of the press,' she said.

'We will work appropriately with media organisations to determine next steps.'

Foreign Minister Marise Payne has reminded Australians not to go to China

Government travel advice website Smartraveller was updated on 7 July to warn Aussies they face the risk of arbitrary detention in China