Contrary to our defence planning, and indeed most of our military history, the army's importance is rapidly shrinking. The air force and navy need some rebalancing in their composition - more of some kinds of weapons, maybe fewer of others - but those services mainly need expansion.

The army, however, needs not rebalancing but reconstruction. And while we think about how to reconstruct it, we should spend much less money on it.

The army is currently fit for a certain purpose - fighting expeditionary land wars, mainly in the Middle East - that has become an unaffordable extravagance as an air and maritime threat to the country rises.

I'm talking about the threat from China, of course.

Just when Australia is finally waking up to its new strategic problem, it is in the middle of a modernisation effort for the army, a costly set of programs that will make it even better at the expeditionary job that is no longer our priority. Those programs should be halted immediately.

For example, to make the army better at fighting in the Middle East, we are planning to spend $18-27 billion on up to 450 infantry fighting vehicles, which would carry soldiers into action and provide supporting firepower.

This is a stupendously large budget, probably exceeding the $17 billion we are paying for 72 F-35s. The F-35s can help defend this country against missile attack. Like other aircraft and the navy's ships and submarines, they can also be used to help the US prevent a Chinese conquest of Taiwan that would lead to a far greater threat to Australia.

Infantry fighting vehicles, on the other hand, would do nothing to deter China in any likely scenario.

Similarly irrelevant are Abrams tanks that we are planning to buy (for at least $2.3 billion) and 29 Boeing Apache attack helicopters ($3.4-5.1 billion) to replace 22 inferior but quite usable Airbus Tigers that are not worn out.

One more such program is the air force's intended acquisition of MQ-9 propeller-driven drones (more than $2.1 billion). Easy to shoot down, these would be just about useless for a war in the western Pacific.

As we look back on recent history, we might think the probability of another Middle-Eastern mission is pretty high. But our participation in such campaigns is never necessary; rather, it is has been merely desirable, to help our allies with important jobs.

If our allies need to intervene again in the Middle East, they will understand if we say: "We can help a bit, but we are not equipped for major ground fighting any more. We're spending on defending our continent instead."

So the first step is to stop programs for modernising the army to do things that are no longer high priorities. The money can be used for strengthening the air force and navy and attending to details of defence resilience, such as hardening bases.

Pushing the army to the bottom of the list of priorities is contrary to Australian military tradition. Our land may be girt by sea, but our great and powerful friends, Britain then the US, have had mighty navies and air forces, so we have contributed to wars mainly with our army.

The US Navy and Air Force are no longer dominant on our side of the world. They could use any air and maritime help we could offer, but would have no use for our ground forces. Also, to attend to safety at home, we need the power to prevent China from controlling the maritime approaches to our continent.

So the second step is to work out how to restructure the army to make it useful for that mission. In part, the answer is to equip it with long-range missiles for threatening ships and any military installations built on nearby islands that China might control.

A program budgeted at $3.2-4.8 billion is supposed to equip the army with rocket artillery - small ballistic missiles for use in ground battles. What's really needed is a large force of much bigger ballistic missiles that can fly at least hundreds of kilometres to threaten such enemy installations as air bases.

The US is developing such missiles and would probably sell them to us. They should become a major feature of our army.

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The Australian Army already wants cruise missiles that would be fired from land against ships. But the program, costed at just $400-500 million, is far too small. We should get hundreds of such missiles.

The role is in fact a revival of a historic army function - coastal defence.

If the army can defend against ships, it might also defend against aircraft and missiles, as the US Army does. Systems for that duty that we are planning to buy for the air force could instead be assigned to the army.

The required shape of the army would change if the US failed to stop China from taking Taiwan and, as is more than plausible, if most of our neighbours responded by submitting to the will of Beijing.

In that case, Australia could find itself on China's front line. This country would again need an army for fighting a sophisticated ground war - in Australia. The mission would be to deter invasion.

But that does not necessarily justify the army's current projects, because expensive ground vehicles are looking increasingly vulnerable. A lot of what the army now wants to buy could be verging on obsolescence.

In a clash over territory last year, Azerbaijan used simple, cheap drones to knock out Armenian tanks. IFVs would be just as vulnerable.

With improvements in sensors, including space sensors, hiding vehicles will become harder. Even if they can be hidden, they may be detected and attacked as soon as they move.

Similarly, attack helicopters are becoming increasingly vulnerable. Since Australia already has such aircraft that don't really need replacing, this is a good time to watch technological development, not rush into an order for new ones.