The short answer is: defend other, vulnerable ships against missiles and submarines, and fire their own missiles against an enemy's ships and land installations.

According to the government's plan, we'll have four classes in service, each with a different mix of those capabilities.

Whether that plan will be fulfilled is doubtful. Australian warship programs this century have usually run late, and construction or modernisation efforts have repeatedly been cancelled or curtailed.

A future government can easily decide a scheme published in 2024 is not what's needed at all.

But let's take Defence Minister Richard Marles's plan at face value and look at what it promises.

Start with three existing ships that will still be with us in two decades, the Hobart class. Their main function is protecting other ships against incoming missiles.

Suppose a group of merchant ships bringing supplies to Australia must pass through waters that could be reached by missile-firing enemy bombers.

An Australian destroyer escorting the merchant ships would shoot long-range interceptor missiles at the weapons that the bombers released.

The enemy weapons could also come from a submarine or a surface ship.

They could come from land launchers, too, so a commander will probably want to keep the convoy at a safe distance from territory where there's a lot of them.

Some such weapons can fly more than 1500 kilometres. If they're cruise missiles, they fly like aeroplanes. Others are ballistic missiles, which zoom into space then descend extremely fast.

The government plans to upgrade the 7000-tonne Hobarts so they can fight both kinds at the same time.

The ships' big limitation is that they have only 48 missile cells.

Each cell can hold a single big interceptor, and some are used for sets of four smaller rounds to fire at targets that get close.

A fierce enemy attack could exhaust the stock.

Greatly worsening that problem is the appearance of extremely cheap, propeller-driven anti-ship missiles, loosely called kamikaze drones. Houthi rebels in Yemen are using such weapons against ships in the Red Sea.

Our next warships are supposed to be economical general-purpose frigates. They're likely to provide much less missile defence than the Hobarts but a better ability to detect and attack submarines. They'll be direct successors to the current Anzac-class ships, which are also GP frigates.

A foreign design, and at first a foreign shipyard, will be used for this set of 11 ships, which can be so numerous because their size and cost should be moderate. The government plans to commission the first in 2029, with construction of the class south of Perth continuing into the 2040s.

The four candidate designs - from Germany, Japan, South Korea and Spain - vary in displacement from 3600 to 5500 tonnes. In general, the bigger a ship and the more elaborate its features, the higher the cost. The Anzacs, which have floated lower in the water as they've been loaded with improvements, displace 3900 tonnes.

The government has cancelled another modernisation and overhaul effort for them, prompting a fall in the size of our destroyer and frigate force by 2026 to a total of nine.

Three of the candidate designs for new GP frigates are already in service with foreign navies and the fourth is an enlargement of one that is. That should reduce program risks.

Costs and risks will rise according to how much the Royal Australian Navy is allowed to customise the chosen design.

Then six Hunter-class anti-submarine frigates are to be delivered from 2034 to 2043 from a shipyard in Adelaide.

As their classification suggests, they should be very strong in detecting and attacking submarines, probably among the best in the world.

A likely special feature, not officially discussed, is extreme quietness, so submarines won't easily detect them.

Satellites will, however. And, however quiet a frigate, any merchant ships it escorts will be noisy.

The Hunters will be incredibly expensive.

Their quietness must be one reason for that, but the main factor is surely that we are changing the original British design by adding a powerful air-defence system, like what's on the Hobarts.

The navy no doubt wanted to do that because of the intensity of the anti-ship missile threat from China.

Yet the 8800-tonne Hunters will have only 32 missile cells.

Lastly, the most eye-catching new design for the navy is the LOSV, which the US Navy is developing.

With very small crews and an ability to sail with no one aboard, the intended mission is for each to carry another 32 missile cells to sea, firing under external command.

So they'll add firepower and disperse it into more hulls, hopefully cheaply.

The design is still immature, so risks are high.

The first of six Australian units is supposed to be delivered from the WA shipyard by the mid-2030s and the last a decade later.

The government has suggested that our LOSVs will particularly add strike power, implying they'll be loaded with Tomahawk missiles for hitting land and sea targets.

But surface-to-air interceptors could go into some or all of their cells instead.

All the classes will be able to carry strike missiles, not necessarily Tomahawks.

Reasons behind that priority are unclear, since air force fighters can attack surface targets more safely than ships can.

Anyway, that's the plan. Now lets see how much of it comes to fruition.