It was unexpected as it was blunt. Let's do a deal.

The Iranian Foreign Minister, Mohammad Javad Zarif, proposed a prisoner swap in a speech to the Asia Society in New York on Wednesday.

The Iranians would release dual UK-Iranian national Nazanin Zaghari-Ratcliffe, who has been convicted of spying and has served three years of a five-year sentence in Iran.

In exchange, they wanted the return of an unnamed Iranian woman who has been held in an Adelaide jail for nearly two years as she fights extradition to the US.

The apparent offer went further than just these two women.

It seemed to extend to other dual nationals currently languishing in Iranian jails — British, Americans and Canadians.

"The United States believes the charges against these people in Iran are phoney," the Iranian Foreign Minister said.

"Fine. Let's not discuss that. Let's have an exchange. I'm ready to do it.

"I have the authority to do it. We've informed the Government of the United States six months ago that we are ready — not a response yet.

"If they tell you anything else, they're lying."

While the issue of dual nationals held by Iran on spying charges is nothing new, few knew Australia was holding an Iranian on US charges over allegedly smuggling restricted US technology to Iran.

The Government in Tehran said Negar Ghodskani was merely a translator, not an Iranian operative.

An Australian Government official confirmed to the ABC the Iranian Government had made regular appeals to our Government for her release.

That same official rejected Iranian media suggestions she was being kept in substandard conditions.

Either way, she has had to spend the past two years in an Adelaide prison during which time she gave birth to a son, who is cared for by her husband.

"We hear about Nazanin and her child," Mr Zarif said.

"I feel sorry for them and I've done my best to help. But nobody talks about this lady in Australia, whose child is growing up apart from its mother."

Nanzanin Zaghari-Ratcliffe is accused of trying to overthrow the Iranian regime, which she denies. ( Facebook: Nazanin Zaghari-Ratcliffe )

How is anyone wanted on a foreign-extradition warrant jailed for so long?

Easily. The appeals process against extradition can, and often does, take years.

And as her applications for bail have been refused for fear she would be a flight risk, she must wait in prison for as long as it takes to exhaust all the appeals processes.

So, as her young son and husband wait in Adelaide, she must remain locked up pending an unwanted flight to the US and, reportedly, up to 20 years in jail.

Imagine the surge of hope for all those foreigners caught in an Iranian jail as the hint of a deal filtered through — and for Ms Ghodskani, who no doubt dared to hope her time in an Adelaide prison might soon come to an end.

But it appears the offer, so publicly and dramatically made, is already dead.

Ms Zaghari-Rafcliffe was convicted of spying and has been in jail for three years. ( Supplied: Amnesty International UK )

At least that was the conclusion of UK Foreign Secretary Jeremy Hunt, who observed his Iranian counterpart, "seems to have somewhat retracted them".

And he blasted the Iranian Government for what he said was the holding of innocent dual nationals like Ms Zaghari-Ratcliffe purely for political leverage.

He said there was a huge difference between Ms Zaghari-Ratcliffe's case and, "this woman in Australia, who is facing due process, proper legal procedure, and she is alleged to have committed a very serious crime".

Australian officials agree what they see as dubious charges against dual nationals in Iran cannot be compared to transparent legal processes here.

What may on the face of it have seemed like a simple swap, is not.

And it is unlikely any of those stuck in Iranian or Australian prisons are going home anytime soon.

All of this comes against a backdrop of increasing pressure on Iran by the US, which withdrew from the multination Iran nuclear deal and has just announced it will apply sanctions against formerly exempt countries from buying Iranian oil.

The offer of a prisoner swap may have lifted the hopes of imminent release among family and friends.

But for the foreseeable future, the maelstrom of superpower pressure and the intensity of Iranian intrigue is simply too intense to allow it.