OPINION: The Gillard government has achieved remarkable success this year in steering contentious legislation through a hung parliament. However, its opponents have a final avenue of attack. They will shift the battle on key federal policies from Parliament to the High Court. The coming year may thus bring a raft of legal challenges, including to the new mining tax, plain packaging for tobacco products and any gay marriage law.

This represents more than a strategic shift by the government's opponents. It also results from new federal-state relationships. The formation of Liberal Party-led governments in Western Australia, Victoria and NSW has created a new willingness to assert state interests and their freedom from federal interference.


The Victorian government has kept open the prospect of a High Court challenge to the proposed federal poker machine reforms. The Deputy Premier, Peter Ryan, has reasserted poker machines are a state responsibility.


Despite the clear political intent, I would be surprised to see a challenge. The Gillard government will likely base its poker machine legislation on the federal corporations power, which is the same power that underpinned John Howard's Work Choices legislation. Howard's success in defending his law in the High Court in 2006 provides a strong basis for a new federal poker machine law.


Another case with low prospects of success is the challenge already filed to Australia's tobacco plain packaging laws. British American Tobacco will argue the constitution does not permit plain packaging for cigarettes unless there is full compensation for the loss of brands and trademarks. The claim is based on the fact the constitution says federal laws giving rise to an ''acquisition of property'' must grant ''just terms''.


It is hard to see how British American Tobacco can win this case. The federal law certainly prevents the company from using its intellectual property, but this is not the same as those trademarks being ''acquired'' under that law.


Next year may bring a vote on changing the Marriage Act to permit gay unions. The debate has proceeded on the basis that the cause will be won or lost in the Federal Parliament. In fact, the High Court will have the final say.


The constitution grants the Commonwealth a power to pass laws on ''marriage'', but it is up to the High Court to determine what this word means. It has not yet decided whether ''marriage'' in the constitution means the union of a man and woman, as it did when the constitution was drafted in the 1890s, or something broader that could now encompass gay marriage.


The High Court may well adopt a broad interpretation and so uphold any federal law for gay marriage. However, if it takes a contrary view, this would end the debate over Australia having a national law on the topic. Gay marriage could still be recognised, but only by state parliaments.


Other cases are waiting in the wings. The High Court has yet to hand down its decision on the Commonwealth's school chaplaincy program. Legal challenges may be mounted to mandatory sentencing for people smugglers and the new federal mining tax. If a legal weakness can be detected in the carbon tax, it is also sure to end up in court. In the longer term, the South Australian Premier, Jay Weatherill, has flagged a challenge to the Murray-Darling Basin Plan.


In all of the challenges likely to be brought next year, the Commonwealth will start in the stronger position. It will have the advantage of drafting the legislation under attack, and will have done so in a way that maximises its chances of withstanding High Court scrutiny.


However, as the recent striking down of the Malaysian asylum deal demonstrates, the High Court cannot be taken for granted. If it detects a constitutional weakness, legislation will be struck down and government action frustrated.


The problem for the Gillard government is that weaknesses may only emerge after a careful, forensic examination of the law by the seven independent judges of the High Court. Other problems can result from the court changing its approach to the constitution in a way that renders a safe law suddenly vulnerable.


All this makes constitutional challenges notoriously difficult to predict. It is why the Commonwealth is always concerned when one of its laws is attacked, even if it has legal advice indicating its position is secure.


George Williams is UNSW's Anthony Mason Professor of Law.

This opinion piece first appeared in the Sydney Morning Herald.