King and her partner Mark Firkin are travel bloggers. They were arrested after allegedly flying a drone near Tehran. They were shooting images for a blog and YouTube video series.

Academic Kylie Moore-Gilbert is also being detained in the country.

Thompson, of Dungog, spent a month in Iran during the major crackdown of 2009 when the regime was "struggling to contain public anger over a presidential election that seemed rigged".

"It was field work for Running With The Blood God, my book about people in a bunch of countries pushing back against their society's limits in order to have some self-respect in a world that unceasingly seeks to strip us of it," he said.

Thompson, a former Sydney Morning Herald journalist, said Iran in 2009 was terrifying.

Mass arrests, shootings and senseless beatings were happening.

A contact who had been in the regime's notorious paramilitary unit, the Basij, told him about the country's surveillance system.

"He told me I was 100 per cent being watched and urged me to get my wife back in Australia to change all my email passwords to something I wouldn't know.

"Otherwise, he said, when I tried to leave the country they would almost certainly take me away and do whatever it took to extract my passwords.

"Any sign of being in Iran as a journalist would seal my fate as a 'spy'."

This led Thompson to suffer intense paranoia.

"I threw up at one stage. Before heading to the airport, I converted all my notes into what looked like poetry - something most Iranians respect," he said.

He apologised to Iranian friends for leaving and being unable to carry out CDs of atrocity footage. He destroyed anything incriminating.

"When I was free and clear, I wept."

Before he went to Iran, he came up with a cover story.

"To get a visa to Iran, you have to present a reason for visiting. Under the crackdown, all foreign journalists had been shown the door.

"I kind of ignored the turmoil and asked for a tourist visa, saying that my aim was to see traditional Persian wrestling.

"Although I never made it to a gym, the country as a whole was locked in a brutal wrestling match."

At a party that Thompson attended in Iran, where people were passing around cups of bootleg shiraz, word came through that a friend of the guests had just been jailed for six years for talking to a European reporter about the unrest.

If Thompson had been detained, he would have been in a notorious jail that the Iranians call Hotel Evin, where the trio of Australians are possibly being held.

While in Iran, Thompson said he was "moved to tears by the pride and bravery of so many Iranian people who are proud to be Persian".

He said these people hold "many wonderful qualities of their culture and history".

Almost every Iranian he spoke to said they do not want a solution imposed from the outside for the future of their country.

They pointed out that the last time they had a democratic government, the US overthrew it and installed the Shah, who "also used violent oppression and kept the Hotel Evin full".

With the subsequent revolution against the Shah resulting in the rule of the Ayatollahs, the people were "done with both foreign interventions and fully fledged revolutions".

"The problem is internal and needs an internal evolution to a just and decent political system."

Regular people in Iran were "suffering badly under sanctions, especially since Trump junked the nuclear deal that Obama had helped make and ratcheted up the sanctions".

Hundreds of thousands of Iranians were killed in a war with Iraq that lasted most of the 1980s and is notorious for the use of poison gas.

"These are not people who buckle. So while Iran is taking a hammering financially and diplomatically, the ace forever up its sleeve is the resilience of its people.

"While the regime in Iran is undoubtedly vile in many ways, it is absurd to keep slamming Tehran as responsible for global terrorism."

The Iranian regime adheres to Shiite Islam, "whereas al Qaeda, Islamic State and the great bulk of jihadist militant groups around the world, from the Taliban in Afghanistan to the Abu Sayyaf Group in the Philippines, adhere to a hyper zealous form of Sunni Islam".