To understand the deep institutional anxiety about Huawei among Australia's closest security partners, you have to understand the British experience with the Chinese telecommunications giant.

It's not an episode that's much advertised, deliberately so. But it's an episode that is held against Huawei when it comes to why it should be denied access to Australia's 5G network, notwithstanding its expertise and industry excellence.

What's on the public record is heavily redacted, as much to hide the Brits' gross embarrassment about the extent of its network compromise as it is on grounds of national security.

The story begins in 2005 when BT (formerly British Telecom) embarked on a 10-billion-pound ($17 billion) upgrade of its network.

Huawei was contracted by BT to supply routers, transmission and access equipment.

For Huawei — which was founded 30 years ago by Ren Zhengfei, a former officer of the People's Liberation Army — the British contract was something of a PR coup, given its goal of breaking into the market in the United States.

Huawei has always denied direct links with the Chinese Communist Government, insisting it is 100 per cent owned by its employees, and the company cites the British contract as evidence of its trustworthiness and reliability.

But Huawei's involvement in the BT upgrade was far from celebrated.

Brits left to 'shut the stable door after the horse has bolted'

BT was under no obligation to inform the British government prior to awarding Huawei the contract.

As the UK's Intelligence and Security Committee reported in June 2013: "It means that the government may not be made aware of contracts involving foreign companies from potentially hostile states until they have already been awarded.

"The government is therefore sometimes put in the position of trying to shut the stable door after the horse has bolted."

In fact, the stable door was now controlled by Huawei.

But it wasn't until 2010, five years after the company was awarded a contract to supply transmission equipment, that the British government raised concerns with Huawei about concerns that its equipment was being exploited.

Sources briefed by British intelligence have told the ABC that a problem was detected inside so-called "core switches" installed by Huawei. These devices are the proverbial stable door for information, letting data in and out.

BT noticed these core switches were doing a lot of "chattering" — to whom they weren't sure, but it was concerning enough for the company to be hauled in by UK authorities.

'The Cell' called in to track down malicious code

The Government Communications Headquarters, the British intelligence and security organisation, established a Cyber Security Evaluation Centre (also known as "the Cell") to study "every piece of hardware or software destined for the UK market" at Huawei's expense.

The Cell also randomly sampled new hardware and software updates destined for UK infrastructure, looking for "malicious code".

In 2011, BT and British government security chiefs flew to Huawei's Chinese HQ in Shenzhen to tell the company they'd identified issues with its equipment.

The extent of the vulnerabilities exposed in BT infrastructure is not public, but the ABC understands it caused BT to replace many of the core switches.

The Brits' Huawei experience saw its Five Eyes security partners — the US, Canada, Australia and New Zealand — harden their approach to critical infrastructure.

Huawei loses out on NBN, 5G now looks unlikely

Australia banned Huawei from the National Broadband Network in 2012 and earlier this year the CIA, FBI and National Security Agency told the US Senate Intelligence Committee they would advise Americans against using Huawei products.

And Australia's recent intervention in the Solomon Islands, bumping Huawei as supplier of a 4,000-kilometre undersea communications cable between Honiara and Sydney, was a national security play camouflaged as foreign aid.

Australian security experts say 5G, which is predicted to start replacing fixed-line telephony, will be so powerful in reach and application it must be afforded maximum protection from sabotage and espionage.

China has an established track record of cyber attacks and Article 7 of China's National Intelligence Law states that "any organisation or citizen shall support, assist, and cooperate with state intelligence work according to law".

Huawei furiously asserts its independence from the Chinese state, but has not been able to shake suspicions from the Five Eyes intelligence partners.

"Forget Beijing's Belt and Road strategy of building ports, road and rail, the Chinese are actively colonising the fifth estate, which is cyber," a security source told the ABC.

"Security is not meant to be convenient, it's meant to protect."

And Huawei, which has one of the few high-functioning, enterprise-level 5G networks, will remain on the outer.

For Malcolm Turnbull it is a terrible dilemma.

The Prime Minister wants to improve relations with Beijing but knows that denying 5G entry to Huawei, one of China's greatest international success stories, will put that mission in further jeopardy.