The government's choices as it considers a new surface ship plan

As it reviews our shipbuilding plan, the government must consider, for example, that in a war the combat ships would escort merchant ships to and from Australia, particularly so we could get important supplies. In another mission, our destroyers and frigates - or little corvettes, if we buy them - might protect landing ships carrying the army to some foreign shore.

The combat ships could also be used in our northern waters to protect Australia against Chinese forces coming south. And they could be sent to help the US Navy in its struggle in the Western Pacific against mighty anti-ship forces projected from Chinese territory.

In all those cases, the weapons threatening the ships and whatever they were escorting would mainly be anti-ship missiles (fired by aircraft, ground launchers or submarines) and submarine-launched torpedoes. The closer our ships moved towards China, the bigger the danger; at a distance, the air- and ground-launched threat would diminish, leaving submarines as the main problem.

We currently have 11 ships for such missions: three destroyers of the Hobart class that displace 7000 tonnes when fully loaded and eight frigates of the Anzac class of 3600 tonnes. The Hobarts are new; the Anzacs will begin reaching the conventional (but elastic) retirement age for a warship, 30 years, in 2026.

That's where the Hunter class frigates come in, to replace the Anzacs. There are supposed to be nine of them, of 8800 tonnes.

But how many will be built and in what configuration is up in the air as the government reviews the whole surface-fleet plan. A decision on exactly what ships we'll construct for the 2030s and beyond is due next year.

British defence group BAE Systems is still preparing for Hunter class construction in Adelaide and does not yet have a contract to build even the first batch of three ships. Also, BAE has offered to change the design if the government so desires.

"Hunter is an inherently adaptable design and the exact configuration of its armament can be modified in later batches of production," a spokesperson says, implying that changes could be implemented after at least three frigates were built to the original specification.

Spanish rival Navantia, designer of the Hobart class, proposes that Australia could order three more Hobarts with improvements. Another possibility is reducing the fleet of big ships and adding a larger number of small ones. The government might, for example, cut three frigates and add six corvettes.

As currently planned, the main combat capability of the Hunters will be detecting and sinking submarines, which is why they're designated as frigates. China has 60 submarines and is expected by the Pentagon to have 80 by 2035.

Optimisation for anti-submarine warfare, including probably world-leading silencing, is one likely reason for the high cost of the Hunters.

Another is that the navy decided that the design, unlike the British Type 26 that it's based on, needed a powerful long-range air-and-missile defense capability, in this case provided by the famous US Aegis system. Such a capability is usually a destroyer feature and was no doubt added because the Chinese missile threat was expected to be severe.

Further complicating things, Australia chose to use not a standard US radar outfit with Aegis but, rather, sensors built by CEA Technologies.

That government-owned Canberra company has a high reputation in the radar world. Its radars probably improve chances of a Hunter hitting incoming enemy missiles, but fiddling with Aegis must have added considerable expense.

And one more reason for the Hunters' high cost will be that the former Coalition government, supported by Labor, insisted that they be built in Australia.

Cost is perhaps the strongest criticism of the Hunter program. The latest estimate, from 2020, is $45.6 billion. Even allowing for inflation and the Australian defence habit of counting all costs needed to make equipment operational, the average of $5.1 billion per frigate looks outrageous.

Britain ordered five Type 26s from BAE Systems at Glasgow last year for 840 million pounds ($1.6 billion) each, presumably not including such necessary extras as ammunition.

A second criticism is that the Hunter program is running late, partly because the Type 26 design was not settled when the navy chose it and began changing it. The first ship was originally supposed to enter service "in the late 2020s"; now the target is 2032.

Offering an alternative to building Hunters, Navantia says it could deliver all three improved Hobarts by 2030.

The Hunter class is also criticised for inadequate armament. Although it has the expensive Aegis system, the hull is fitted with only 32 cells for the associated interceptor missiles.

The missile capacity may not be so unreasonable for a primarily anti-submarine design. But critics nonetheless note that a Hobart has 48 such cells and a US Arleigh Burke class destroyer of 9900 tonnes has 96.

Why does it matter? Because an intense anti-ship missile attack, or a succession of attacks, could easily exhaust a Hunter's interceptor stock, leaving it defenceless against further salvoes.

The number of cells is hardly a definitive measure of the value of a warship. Nonetheless, BAE and Navantia have evidently sensed that the navy may regret putting so few in the Hunter design.

BAE's proposed Hunter derivative has 96. An important anti-submarine feature, a sensitive sonar array trailed behind the ship, reportedly would have to be deleted, however.

Navantia is offering a new design with a whopping 128 cells. Since it has not been developed in detail, it presumably would not be built until after the proposed extra Hobarts.

Another way of getting more capability to sea could be to build a larger number of smaller ships. These would be the tier-two ships called for by the Defence Strategic Review, published in April.

Smaller ships would be more vulnerable, but for more dangerous missions they could accompany Hunters and Hobarts. Moreover, by splitting capability into more hulls, the navy would have more flexibility in deployment and more assurance that something would still be afloat after an enemy attack.

German builder Luerssen, designer of Australia's forthcoming Arafura class patrol vessels (which hardly count as warships), has offered a design for a 2300-tonne corvette with 16 cells. Navantia and partners have proposed building six well-equipped light frigates that would have 16 cells and be as big as the Anzacs.

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Another German builder, TKMS, is offering a 4700-tonne design with 32 cells and plenty of other bells and whistles. That looks like a possible replacement, not supplement, for the Hunter program.

The decision will not be easy. Hunter development troubles should be close to resolution, in which case the obvious thing to do is promptly begin building. But Navantia is offering big ships faster.

The navy wanted anti-submarine ships, and in that role the Hunters should be excellent. Yet a reason for conducting the review is that Australia will eventually have nuclear submarines, which will bring a new anti-submarine capability.

Finally, building many little ships would create flexibility - but not if they are too weak to deploy where we need them.