Uniform Civil Procedure Rules 1999

In a proceeding, 2 or more persons may be plaintiffs or defendants or applicants or respondents if—

separate proceedings were brought by or against each of them and a common question of law or fact may arise in all the proceedings; or

all rights to relief sought in the proceeding (whether joint, several, or alternative) arise out of the same transaction or event or series of transactions or events.

Also, in a proceeding, 2 or more persons may be defendants or respondents if—

there is doubt as to—

the person from whom the plaintiff or applicant is entitled to relief; or

the respective amounts for which each may be liable; or

damage or loss has been caused to the plaintiff or applicant by more than 1 person, whether or not there is a factual connection between the claims apart from the involvement of the plaintiff or applicant.