An Australian journalist detained in China for weeks is being held on national security grounds, China has said.

Cheng Lei, a presenter for China Global Television Network (CGTN), is suspected of “criminal activity endangering China’s national security”.

She has been detained since August 14.

The announcement comes after the last two Australian journalists working in China flew home to Sydney after a five-day diplomatic stand-off.

The Australian Broadcasting Corporation’s (ABC) Bill Birtles and the Australian Financial Review’s (AFR) Mike Smith landed in Sydney on Tuesday.

Chinese authorities questioned both men before their departure.

The ABC reported Birtles was “not asked about his reporting or conduct in China” but instead about Ms Cheng.