This term is used loosely. It was discussed by McHugh J in Ousley v R (1997) 192 CLR 69, page 99:
A collateral attack on an act or decision occurs when the act or decision is challenged in proceedings whose primary object is not the setting aside or modification of that act or decision[73]. In In re Preston[74], however, Lord Scarman used the term “collateral challenge” to include any process challenging a decision – including an application for judicial review – other than a proceeding by way of appeal. This use of the term is readily intelligible. However, with the widespread availability of judicial review procedures, it conduces to clarity of thought, in my opinion, if the term “collateral challenge” is confined to challenges that occur in proceedings where the validity of the administrative act is merely an incident in determining other issues[75].
The Supreme Court of Victoria is a superior court of record. Judicial orders of superior courts cannot be made the subject of a collateral challenge. Such orders are valid until they are set aside or varied by appeal to a higher court even if they have been made in excess of jurisdiction[76]. They cannot be treated as nullities[77]. It is not open to an inferior court to ignore them. Indeed, a superior court “which, after a real trial, has given a valid decision determinative of right, liability or status, has no jurisdiction to recall it whatever mistakes may have been made in facts or law”[78]. If warrants issued under the Act by the Supreme Court are judicial orders, it follows that the trial judge was correct in holding that he had no jurisdiction to entertain a collateral challenge to their validity.
These comments were adopted in Singh v Minister for Immigration [2016] FCCA 387, [25] where Manousaridis J observed:
[25] … The expression “collateral attack” is often used to denote the challenging of an administrative act or delegated legislation “in proceedings where the validity of the administrative act [or delegated legislation] is merely an incident in determining other issues”.12 An example is Guo v Minister for Immigration and Citizenship where Besanko J considered whether a regulation made under s 504 of the Act was invalid.13 His Honour noted that the Minister did not suggest that a collateral attack on the validity of the regulation was not permissible, and that the appeal was conducted on the footing that if his Honour were to decide that the regulation in question was invalid, constitutional writs directed to the Tribunal should issue.14
Collateral challenges may nonetheless be permissible–the observations in Ousley are in relation to decisions made by a superior Court of record. Decision made by a Tribunal or officer of the Commonwealth or a State are different. Thus, in Robinson v Vanston [1999] VSC 541, [152], Ashley JA observed:
Focussing, then, on the matter of principle, the making of a decision upon an application for classification is an administrative act. Prima facie it should be open to challenge by administrative law process. That should not prevent it being subject to collateral challenge, as to which see Ousley v The Queen (1997) 192 CLR 69 at 98- 99 per McHugh J.
More recently, in R v Debono [2012] VSC 350; 222 A Crim R 194 Kyriou J observed at [162]:
Administrative acts and decisions are generally susceptible to collateral challenge in a court.[151] For example, a collateral challenge to the validity of a telecommunications interception warrant can be brought in the course of a trial, provided that the trial judge has jurisdiction to determine such a challenge.[152] However, the scope of permissible collateral challenge is unclear.[153]
Sometimes statute may limit the ability to mount a collateral attack. In Director of Housing v Sudi [2011] VSCA 266; 33 VR 559 the Victorian Court of Appeal considered a situation where the Director made an application to VCAT for eviction orders under the Residential Tenancies Act. VCAT considered an argument that the anterior decision by the Director to make such an application at all was unlawful because of the Charter of Human Rights and Responsibilities. Maxwell P considered the issue from [58] onwards and concluded that the particular statutory scheme did not allow VCAT to uphold such a collateral attack.