Alo-Bridget Namoa, 23, was found guilty by jury for conspiring with her husband Sameh Bayda to do an act in preparation for a terrorist act in December 2015 and January 2016.

The NSW Supreme Court jury was told the then-18-year-olds married in an Islamic ceremony before Bayda's planned "attack" on non-Muslims on New Year's Eve 2015.

Both before trial and on appeal, Namoa had argued there was a common law rule that spouses alone could not conspire.

But the High Court on Tuesday held that her conviction was valid as a husband and wife were each a "person" and could be found guilty of the conspiracy offence.

"Whatever may have been the historical position, there is no longer any principle in Australian common law respecting the single legal personality of spouse," Justice Jacqueline Gleeson said in written reasons on Tuesday.

"Senior counsel for the appellant properly acknowledged that the common law rule for which he contends cannot depend upon any proposition that husband and wife form a single person."

Namoa had cited cases from the UK, Canada and New Zealand that gave various explanations for the special position of spouses in relation to the crime of conspiracy.

But Justice Gleeson said the courts in those international cases hadn't been concerned with the meaning of conspiracy.

Further hurting Namoa's case was a document produced when drafting the federal criminal code in the 1990s that stated there was no protection provided for spouses regarding conspiracy and that marital immunity was "outdated".

Justice Gleeson's reasons were supported unanimously by the court.

Namoa's jail term of three years and nine months expired in 2020, but she's been subject to a federal control order since.

She is due to be arraigned on Friday in the District Court over an alleged breach of that order last July.

Australian Associated Press