SYDNEY • A self-described Australian "Islamic Bonnie and Clyde" couple were yesterday charged with planning a terrorist act, which reportedly involved a Sydney stabbing attack.

Sameh Bayda and Alo-Bridget Namoa, both 19, were already in custody, accused of collecting documents likely to facilitate terrorist acts.

The charges followed an extensive investigation by officers from the New South Wales joint counterterrorism team, state police said in a statement, adding: "The charge carries a penalty of life imprisonment."

The Sydney couple, reportedly husband and wife, were arrested about a year ago.

At the time, Bayda was allegedly found with Arabic documents on how to carry out a stabbing and how to make an improvised explosive device (IED), The Sydney Morning Herald said.

Namoa, a Muslim convert of Catholic Tongan heritage, allegedly had in her possession an Islamic flag and a hunting knife, as well as instructions in Arabic on making an IED detonator, the Australian Broadcasting Corporation (ABC) reported.

ABC also said that in a text message conversation with Bayda, she described them as the "Islamic Bonnie and Clyde".

Her lawyers have claimed that she was merely a "terrified young woman" whose refusal to answer questions was driven by fear.

But in September, a court heard that Namoa had an "infatuation with violence" and said she could watch beheadings "all day".

Both were denied bail yesterday and the case is due back in court on March 15, a Central Local Court official in Sydney told Agence France-Presse.

Canberra has become increasingly worried about home-grown extremism. The terror threat level was raised in September 2014.

Australian officials said they have prevented 11 terror attacks on home soil in the past two years. But some have succeeded, including the murder of a Sydney police employee in 2015 by a 15-year-old.

Counterterrorism police have made a series of arrests since late 2014, with the young age and radicalisation of many of those detained a growing concern for the authorities.