From 1 November 2009, witnesses in a broader range of relationships may be excused from giving evidence in criminal proceedings.
Right now s 400 of the Crimes Act 1958 provides that only the wife, husband, mother, father or child of an accused person can ask a court to excuse them from giving evidence for the prosecution.
Some courts consider that provision extends to de facto and same-sex relationships. Others consider that only legally married spouses can apply for that exemption. (I can’t find any conclusive judicial authority on the point, but in R v RGP (2006) 167 A Crim R 468;  VSCA 259 at  –  the Court of Appeal commented that s 400 shouldn’t be interpreted broadly. In that case, the Court considered s 400 did not apply to step-children.)
The Full Bench of the Federal Court considered s 400 did not apply to former spouses or to de facto partners: S v Boulton (Examiner, Australian Crime Commission) (2006) 162 A Crim R 489;  FCAFC 99 per Black CJ at .
Under s 18 of the new Evidence Act 2008, spouses, de facto partners, parents or children of an accused person may ask to be excused from giving evidence.
De facto partner, parent and child are defined in the dictionary to the Act. For parents and children, they include adoptive and step-relationships.
In the NSW and Commonwealth versions of the Evidence Act, certain offences are excluded from the operation of s 18. (Mostly sexual and family violence offences, when an accused person might improperly influence a complainant to withdraw their complaint.)
Section 19 is not enacted in the Victorian Evidence Act, so witnesses can ask to not give evidence in any criminal proceeding. But, the court must consider factors specified in sub-ss 18(6) and (7), which would justify refusing to excuse a witness from giving evidence if the court was satisfied they were being improperly influenced to recant.