Three Chinese naval vessels have been sailing in international waters off Australia's east coast and conducting live-fire exercises, causing trans-Tasman flights to divert.

The fleet consists of the Jiangkai-class frigate Hengyang, the Renhai-class cruiser Zunyi and the Fuchi-class replenishment vessel Weishanhu.

The New Zealand Defence Force has been working with its Australian counterpart to monitor the ships since Friday.

On Tuesday, the Defence Force said the ships were sitting 218 nautical miles east of Tasmania.

The Jiangkai-class frigate Hengyang Photo: ABC / Department of Defence

What vessels are sailing in the flotilla?

As outlined above, the fleet consists of a Jiangkai-class frigate, Renhai-class cruiser and Fuchi-class replenishment vessel.

These classes fall within the US warship classification system, which uses Chinese-origin names and has been adopted by many industrialised countries.

However, Beijing categorises vessels with numerals instead of names.

Under China's classification system, the Hengyang is a Type 054A frigate, the Zunyi a Type 055 destroyer and the Weishanhu a Type 903 replenishment vessel.

Why is one vessel categorised differently in the two systems?

Modern naval vessels are generally classified by the volume or weight of the water they displace.

Warships with a displacement weight exceeding 10,000 tonnes are classified as cruisers.

According to public data, the displacement weight of the fully loaded Zunyi exceeds 12,000 tonnes, which means the U.S. Department of Defense and the Office of the Secretary of Defense have classified the vessel as a cruiser.

But China classifies the Zunyi as a destroyer, a category typically reserved for warships that have a smaller displacement weight than cruisers.

Some analysts believe that Beijing deliberately "downgrades" classifications to maintain a low defence profile.

China maintains the ship is primarily involved in defensive activities close to home, which are typically handled by destroyers.

Cruisers, meanwhile, are designed for long-range patrols and offensive naval duties.

However, the fact that Chinese warships conducted live-fire drills far from mainland China suggests Beijing may be abandoning its traditionally defensive classification of Type 055 vessels.

Indeed, Chinese military enthusiasts call Type 055 vessels "10,000-tonne mega-destroyers".

The New Zealand Defense Force sent the HMNZS Te Kaha to monitor the Chinese fleet.

With a displacement weight of 3,660 tonnes, the Te Kaha is a frigate of the same class as the Hengyang, which has a displacement weight of around 4,000 tonnes.

Are naval forces becoming more aggressive?

Compared to modern destroyers, cruisers appear to be becoming an increasingly outdated category of vessels.

The boundary between them is blurring, with destroyers becoming increasingly aggressive.

The World War II-era battleship is a modern example of a military naval vessel that became obsolete with time.

And as modern naval warfare evolves, traditional classification boundaries are becoming blurred.

Whether Type 055 vessels can be classified as cruisers or destroyers, their combat effectiveness increasingly relies on "beyond-visual-range strikes".

New Zealand's naval frigate HMNZS Te Kaha. Photo: SUPPLIED / US NAVY

What weapons do Renhai-class cruisers typically carry?

Data from the United States Naval Institute suggests Renhai-class cruisers are typically armed with 112 universal vertical launch system missile tubes, with 64 launchers fitted forward in an 8x8 configuration, and 48 launchers further aft in a 6x8 configuration.

The universal missile launch systems can accommodate different types of projectiles depending on operational needs, including:

• HQ-9B long-range surface-to-air missiles with an attack radius of 185 km.
• YJ-18 anti-ship cruise missiles with a range of 535 km.

Reports suggest the large vertical launch systems may also be capable of launching anti-submarine missiles and could potentially be upgraded to carry land-attack cruise missiles or even anti-ship ballistic missiles in future.

According to China's state-run Ta Kung Pao newspaper in Hong Kong, Renhai-class cruisers are already capable of carrying the latest YJ-21 anti-ship missiles.

This variety of hypersonic missile has a range of 1000 km and has been described by Chinese state media as a "carrier killer".

Beyond missile capabilities, Renhai-class cruisers are equipped with a 130mm H/PJ-45 naval gun at the bow, 30mm weapon systems for short-range defense and two triple torpedo launchers for anti-submarine warfare.

Naval guns at the bow are typically the last line of defense for warships in close-range engagements.

Personnel aboard the HMNZS Te Kaha reported observing the Renhai-class cruiser Zunyi's 130mm naval gun on the bow firing during the exercises.

The decision to conduct live-fire drills using a defensive weapon rather than offensive missile systems may indicate an attempt to avoid excessive international criticism.

Are the vessels nuclear-powered or able to launch nuclear weapons?

All three Chinese vessels are conventionally powered.

The Renhai-class cruiser Zunyi is powered by four gas turbines, the Jiangkai-class frigate Hengyang by four diesel engines and the Fuchi-class replenishment vessel Weishanhu by two diesel engines.

Currently, there is no evidence that the Zunyi is capable of launching nuclear weapons.

However, modern nuclear weapons differ significantly from those of past decades.

Modern nuclear weapons can be miniaturized and installed on various types of missiles, with intercontinental ballistic missiles being the most common delivery system.

On 25 September, China's defence ministry announced a successful test launch of an intercontinental ballistic missile into the South Pacific.

Such long-range missiles can carry different types of warheads, including nuclear or conventional payloads, depending on mission requirements.

Has China followed international law in carrying out naval exercises near Australia?

It depends on who you ask.

In the context of marine navigation, countries primarily refer to the United Nations Convention on the Law of the Sea when working out whether another nation is acting within international law.

Under the convention, article 19 prohibits foreign military exercises within a coastal state's territorial waters. No such restrictions apply to the high seas.

The key legal basis for warships sailing in international waters is detailed in article 87 (freedom of the high seas), article 90 (right of navigation) and article 95 (immunity of warships on the high seas).

So far, so good.

However, article 88 also states that the "high seas shall be reserved for peaceful purposes".

The International Tribunal for the Law of the Sea, which rules on disputes in international waters, would need to decide whether the exercises were peaceful in nature.