For any other program, of even modest size, it would lay out the justification and the detailed reasons for rejecting alternatives. Experts would comment on the plan, perhaps criticizing it, and journalists would report on the debate.

Yet for this colossal (but overly slow) spending effort, we're getting only vagaries.

That's because stating the concrete reasons would only worsen our relations with Beijing. These are weapons for deterring China from starting a war, which means they are weapons that, if necessary, we we would use against China.

Worse, if we insisted on the government getting into the nitty-gritty about why we had to have nuclear submarines, it would start reeling off descriptions of them hunting and torpedoing Chinese vessels (and therefore killing crews), lobbing missiles at some Chinese targets or helping friendly forces to attack others.

We don't want that stuff ventilated by ministers.

That leaves the public relying on explanations from such people as strategic analysts, retired naval officers and, to the extent we're up to the job, journalists.

Regardless of who explains it, the details are unpleasant: it's war talk. But if you really want to know why we're spending this money, there's no alternative. So here's a summary.

Australia must worry about two main war scenarios, the first being the possible Taiwanese War. We would be highly likely to help the US defend Taiwan against China, because we would fear that if the island were lost we would end up in the second, nightmare scenario.

In that one, which we can call China on Our Doorstep, a China that had conquered Taiwan would be dominating a helpless East Asia and looming over us from bases in such places as East Timor. Maybe we would not have the US to help us.

In both scenarios, we must expect China to attempt to block our foreign seaborne supplies, including critical stuff such as fuel. To sink our merchant ships far from its bases, China would use submarines, which we would try to find and destroy with aircraft, surface warships and our own submarines.

Except when loitering in choke points, such as Indonesian straits, diesel-electric subs have low value in such an anti-submarine mission, simply because they are so slow. They can sprint, but over long distances they typically move at about 8 knots (15 km/h) to conserve battery charge and avoid sticking up a detectable breathing pipe when running diesel generators.

A nuclear submarine moves fast. If a target has been detected hundreds of kilometres away, the nuke may lunge towards it at perhaps 25 knots, slowing down on approach to make less noise, avoid detection and listen for a fix.

Maybe the target is itself a fast-moving nuclear submarine, and the Australian sub's task is to track it while an aircraft flies in for the attack. A diesel-electric sub just could not keep up with the target.

Defence Minister Richard Marles alludes to such operations when he says the nuclear submarines would protect our trade.

This does not mean they would sail alongside merchant shipping in a convoy. Rather, their anti-submarine operations would impose a cost - a risk of destruction - on Chinese vessels attempting to blockade us, says Christopher Skinner, a retired naval officer and advocate for nuclear submarines for Australia.

So far, so defensive. Then there is aggressive use of nuclear submarines - sending them forward to seek and destroy enemy vessels that are not blockading us. This mission is more likely to arise in the desperate China on Our Doorstep scenario.

Protecting ships, including merchant vessels, would be a distraction for Chinese commanders, forcing them to disperse resources.

Those officers might know that an Australian nuclear submarine is out there somewhere, but where? Maybe it revealed its location by sinking a Chinese ship eight hours ago, but where is it now? Comfortably moving at 20 knots, it could be anywhere within a 300 kilometre radius.

The threatening nuclear sub is not likely to go home soon, because it has virtually unlimited fuel and, being fast on its return voyage, can delay its departure until it has maybe only a week's food left on board.

"As a corollary of their speed, stealth and endurance, a nuclear-powered submarine puts the biggest possible question mark in the mind of any potential adversary," Marles told parliament this week. "This is a capability that will make Australia a more difficult and costly target for anyone who wishes us harm."

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Question marks appear over Chinese land targets, too. Air defences may be able to protect them from strike aircraft, but a nuclear submarine could get close enough for a missile attack, anyway - then speedily move away, to avoid reprisal.

In the China on Our Doorstep scenario, our submarines could be critical in interrupting supplies and reinforcements to nearby Chinese bases.

Although a general invasion of Australia would be highly improbable, landings aimed at securing bases on our continent may not be. In that case, our subs may be needed to destroy amphibious assault ships.

A key point is that, while powerful countries also use submarines, such vessels are particularly important for weaker powers, simply because they are so hard to find. Smaller countries, such as Australia, have gone for diesel-electric subs, because they're much cheaper - but such vessels have always had the limitation of poor mobility and are now becoming too easy to detect when recharging.

So, yes, we'll be spending an awful lot of money on these nuclear boats. But we'll be getting enormous defense capability from them.