Policy made on the back of an envelope can go awry, and if John Howard pushes ahead with his plans to takeover management of the Murray-Darling Basin Australia may end up with something very different to what the PM has had in mind.

The constitutional law is fuzzy on the subject. However, the PM and Malcolm Turnbull have said they will rely on a “cocktail” of commonwealth powers, including the corporations laws, control of interstate trade and commerce, and the external affairs power.

Of course, Kevin Rudd may win the next election and strike a deal with the Labor premiers, but if he fails and this matter ends up in court, these may be the outcomes.

The corporations laws and control of interstate trade and commerce have their problems.

Farmers and irrigators may choose not to incorporate and run their properties as Joe Bloggs, human being, not as corporations. It also doesn’t cover the State of Victoria, which is not a corporation.

Trade and commerce is undermined by section 100 of the Constitution, which says trade and commerce laws cannot limit the reasonable use of waters for conservation or irrigation.

That forces the Feds back on the external affairs powers, as seen in the Tasmanian dams case. These powers have to enforce an international convention.

In the Franklin case, it was the World Heritage Convention. The feds have cited two conventions about rivers and wetlands in regards to the Murray-Darling.

These are pro-environment conventions. The external affairs power will only help the commonwealth with laws that protect the rivers and the environment – not laws that protect the water rights of farmers and irrigators.

One of Howard’s last acts – one of Howard’s last Acts – may be a gift to the green movement, at the expense of Coalition voters.