A married couple who referred to themselves as the "Islamic Bonnie and Clyde" have been charged with conspiring to plan a terrorist attack.

Sameh Bayda and Alo-Bridget Namoa, both 19, were already on remand for terrorism-related charges when their charges were upgraded to conspiracy to prepare or plan a terrorist act during a hearing at Sydney's Central Local Court today.

Court documents revealed the pair was allegedly conspiring to use a knife or knives to carry out a stabbing attack.

The charge is based on allegations levelled at them when they were arrested early last year.

Bayda is already accused of collecting documents in Arabic containing instructions on how to make an improvised explosive device and how to carry out a stabbing attack.

Namoa, a Muslim convert of Catholic-Tongan heritage, is accused of recklessly possessing a hunting knife and Islamic flag, as well as instructions in Arabic on how to make a detonator for an improvised explosive device.

They have been in custody since the beginning of last year.

Namoa was also previously convicted for refusing to answer questions about her husband's charges in the NSW Crime Commission.

The questions put to Namoa included references to a text message conversation she had with Bayda in which she used the term "Islamic Bonnie and Clyde".

According to court documents, she was also asked about Islamic State propaganda, whether her husband gave her a knife, and whether he was planning to martyr himself.

In a statement, NSW Police said the couple had their charges upgraded as part of an ongoing investigation known as Operation Chillon.

"Today's charges follows an extensive investigation by officers from the NSW Joint Counter Terrorism Team and ongoing consultation with the Commonwealth DPP," NSW Police said.

"The charge carries a penalty of life imprisonment."

Bayda and Namoa did not appear in court during the brief hearing and will remain in custody.

Their case will return to court on March 15.