11/08/2017 10:18 AM AEST | Updated 11/08/2017 1:35 PM AEST

'ANZUS Will Be Invoked' If U.S. Attacked: Turnbull

PM backs U.S. as predecessors argue for a missile defence shield.

Malcolm Turnbull risks "an incredibly costly, dangerous, tragic outcome" if Australia enters a conflict between North Korea and the United States, after the Prime Minister invoked the ANZUS treaty in response to escalating rhetoric between the two nuclear powers.

The warning -- from Hugh White, Professor of Strategic Studies at Canberra's ANU -- comes after Turnbull told 3AW on Friday an attack on either Australia or the U.S. would trigger the agreement, which commits the allies to consult if either is attacked.

"The United States has no stronger ally than Australia," Prime Minister Malcolm Turnbull told 3AW.

"If there is an attack on the United States by North Korea then the ANZUS treaty will be invoked and Australia will come to the aid of the United States, just as if there was an attack on Australia, the United States would come to our aid."

Jonathan Ernst / Reuters
U.S. President Donald Trump meets with Australia's Prime Minister Malcolm Turnbull ahead of an event commemorating the 75th anniversary of the Battle of the Coral Sea, aboard the USS Intrepid Sea, Air and Space Museum in New York, U.S. May 4, 2017. REUTERS/Jonathan Ernst

The ANZUS treaty was last invoked on September 14, 2001, by John Howard, in the wake of the attack on the Twin Towers in New York.

White told the ABC there remains too much uncertainty about America has in mind when it comes to responding to North Korea.

"Therefore I think it was a bit unwise of Mr Turnbull to commit Australia so plainly when it's so unclear of what really America has in mind," he said.

White urged caution, and said we should not leap to the conclusion that we have no choice but to go to war with North Korea.

He said while many scenarios remained on the table, a war with North Korea would be "an incredibly costly, dangerous, tragic outcome".

This could easily be the worst war the world has seen since the Second World WarHugh White, Professor of Strategic Studies at ANU

"That sounds like a very dark thing to say, but it is no exaggeration, but that is a step that needs to be considered, exceptionally carefully, much more carefully than the Prime Minister seems to have done."

North Korea on Thursday threatened plans to fire missiles about 40kms off the U.S. territory of Guam, which is about 4,500km to the north of Australia.

U.S President Donald Trump clapped back with a threat of "fire and fury" against the hermit state, before he doubled down on Thursday by saying his comments may not have been tough enough.

White said he expected a lot of pressure on Australia from China to stay out of any potential conflict.

"That has very big implications obviously for a lot of other Australian interests. Australian political leaders keep on saying, 'We don't have to choose between America and China,' but it is just in this kind of situation that we really do face."

Greens leader Richard Di Natale has has accused the Prime Minister of putting a target on Australia.

"Malcolm Turnbull, by backing in Donald Trump, has just put a target on our back," Di Natale told reporters in Canberra.


Meanwhile, the local share market appears to have reacted in early trade to the strong rhetoric from Washington and North Korea

The Australian sharemarket plunged below 5700 points in early trade, a selloff thatwiped about $23 billion off the market's value. The ASX fell 1.4 per cent at 5681.2 points, with all the main sectors in the red. Wall Street also posted its biggest loss in three months.

CMC chief market analyst Ric Spooner told Fairfax thelatest threats over North Korea had escalated to the point where market was obliged to react.

"This has injected the first note of volatility into US stock marketsfor some time. US markets had previously been becalmed amidst the Goldilocks scenario of strong profit growth, low interest rates and full valuations," he said.

"Difficult to assess political risk is now intruding on this scenario."

KCNA KCNA / Reuters
North Korean leader Kim Jong Un reacts with scientists and technicians of the DPRK Academy of Defence Science after the test-launch of the intercontinental ballistic missile Hwasong-14 in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang July, 5, 2017.

Former Prime Minister Tony Abbott told the Sydney Morning Herald Australia urgently needed to consider a missile defence shield to protect against attack by nuclear-armed North Korea.

"We should be urgently investing in upgraded missile defences," the former Coalition Prime Minister said.

It echoed similar statements made to the paper by Abbott's predecessor the month before.

The former Labor PM Kevin Rudd told Fairfax Australia would be "well advised to begin analysing ballistic missile defence needs, available technologies and possible deployment feasibility for northern Australia."

In April, shortly after North Korea's army held it's "largest ever" live-fire artillery drills, a former national defence advisor to Tony Abbott and John Howard issued calls for a missile defence system.

Getty Images
In this handout photo released by the South Korean Defense Ministry, South Korea's missile system firing Hyunmu-2 missiles into the East Sea during a South Korea-U.S. joint missile drill aimed to counter North Korea's ICBM test on July 29, 2017 in East Coast, South Korea.

There are widely cited intelligence reports indicating North Korea has up to 60 nuclear weapons under its control, and it has successfully produced a miniaturised nuclear warhead that puts it well on the path to being a fully fledged nuclear power.

On Monday the U.N. imposed what Australian Foreign Minister Julie Bishop called "the toughest and most comprehensive package of sanctions against North Korea to date".

More to come.