Retrospective Laws, or ex-post facto law refers to a law passed after the occurrence of a fact, or after the omission of an act which retrospectively changes the legal consequences or relations of such fact or deed.
Article 15(1) of the International Covenant on Civil and Political Rights expressly prohibits the implementation of retrospective criminal laws;
“No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence, under national or international law, at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time when the criminal offence was committed. If, subsequent to the commission of the offence, provision is made by law for the imposition of a lighter penalty, the offender shall benefit thereby.”
Article 15(1) also requires that criminal laws must be sufficiently precise and accessible so as to allow individuals to know in advance whether their actions are legal or not.
Article 15(1) does not apply to civil laws, which can generally be retrospective, though some limits may apply under other provisions of international human rights law.
Article 15(2) clarifies that paragraph 1 does not prevent the trial and prosecution of any person for acts or omissions which, at the time they were committed, was criminal according to the general principles of law recognized by the community of nations. For example, persons may be prosecuted for genocide and grave war crimes regardless of Article 15(1).
African Charter for Human Rights
Article 7(2) of the African Charter for Human and Peoples' Rights states:
No one may be condemned for an act or omission which did not constitute a legally punishable offence at the time it was committed. No penalty may be inflicted for an offence for which no provision was made at the time it was committed. Punishment is personal and can be imposed only on the offender.
The American Convention on Human Rights
Article 9 of the Convention states:
No one shall be convicted of any act or omission that did not constitute a criminal offense, under the applicable law, at the time it was committed. A heavier penalty shall not be imposed than the one that was applicable at the time the criminal offense was committed. If subsequent to the commission of the offense the law provides for the imposition of a lighter punishment, the guilty person shall benefit therefrom.
Arab Charter on Human Rights
Article 15 of the Charter prescribes that no crime and no penalty can be established without a prior provision of the law. In all circumstances, the law most favorable to the defendant shall be applied.
European Convention on Human Rights
Article 7 of the ECHR states:
No one shall be held guilty of any criminal offence on account of any act or omission which did not constitute a criminal offence under national or international law at the time when it was committed. Nor shall a heavier penalty be imposed than the one that was applicable at the time the criminal offence was committed.
This article shall not prejudice the trial and punishment of any person for any act or omission which, at the time when it was committed, was criminal according the general principles of law recognized by civilized nations.
S27 of The Victorian Charter of Human Rights and Responsibilities expressly bans the application of laws retrospectively.
WBM v Chief Commissioner of Police (2010) 27 VR 469,  VSC 219 The applicant was convicted of child sex offences. During the period of his suspended sentence for these offences, the Sex Offenders Registration Act 2004 (SORA) came into effect, causing the applicant’s name to be included in the Victorian sex offenders registry. He sought a declaration that the SORA was inconsistent with the Charter, submitting that its retrospective application to him breached his right under s 27 of the Charter. The court dismissed the application, stating the SORA’s application to the applicant did not constitute an imposition of a "penalty" for the purposes of s 27 of the Charter. It stated that the SORA was concerned with the prevention of re-offending by offenders, rather than their punishment. Hence, there was no violation of s. 27.
In R v Kidman (1915), the High Court held that the Commonwealth Parliament had the power to pass retrospective criminal laws.
In Polyukhovich v Commonwealth (1991), a High Court majority upheld the Commonwealth Parliament’s power to retrospectively criminalise, in Australia, World War II war crimes committed in Europe. Three judges confirmed a general power to pass such laws. Two judges (Deane and Gaudron JJ) denied this power, while Brennan J decided against the government on other grounds. The crucial “swinging” judge was Justice Toohey, who found that the Commonwealth did have such power with regard to the grave moral transgression of war crimes.
Therefore, the scope of the government’s power to pass retrospective criminal laws is not clear. It appears that there is no restriction on the ability of State Parliaments to pass retrospective criminal laws.
United States of America
The United States Constitution expressly prohibits the passing of retrospective laws.
Article 1(9)(3): No Bill of Attainder or ex post facto law shall be passed
Article 1(10)(1): No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make anything but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Canada’s Constitution Act 1982, entrenches the Canadian Charter of Rights and Freedoms.
Section 11(g) of the Charter prohibits a person from being convicted of an offence, unless at the time the act was committed, it was an act proscribed by domestic law, international law, or by the general principles of the laws of civilised nations. Section 11(i) mandates that a person will be given the lesser sentence, where the punishment for a crime has varied since the person’s conviction.
Chapter 2 of the Constitution of South Africa contains the South African Bill of Rights. Section 35(3) of the Bill of Rights guarantees the right not to be convicted under a law which, at the time the offence was committed, was not existent under national or international law. This section also guarantees that the lesser punishment will be ordered for an offence, where the prescribed punishment has been changed between the commission of the offence and sentencing.
This section was applied in the case of Donald Veldman v The Director of Public Prosecutions (2005) in the Constitutional Court of South Africa. In this case, changes to the law coming into effect after the offender pleaded guilty, allowed for an increased penalty. The Court held that the application of the heavier punishment would be unconstitutional and violated the offender’s right to a fair trial under s. 35(3) of the Constitution.