Foreign Minister Julie Bishop will lobby Iran to take back hundreds of failed asylum seekers when she visits Tehran next week.

Iran has not cooperated with forced returns of its citizens who have been rejected as refugees by Australia.

More than 3,000 Iranians in the community on bridging visas are yet to have their claims assessed, and more than a fifth of people in mainland detention centres are from Iran.

Government MP Paul Fletcher said there were many Iranians in detention whose claims had been refused and that returning people found not to be refugees was vital.

"Our objective is ultimately to have those people returned to the country that they came from," he told Sky News.

"The key point is, we have a significant number of people in immigration detention at the moment from Iran.

"Where there are people from Iran and indeed other countries who are found not to have refugee status, then ultimately what we seek to do is have those people returned.

"It inevitably can raise difficulties and challenges, and that is an important reason why it does make sense to be having discussions with the countries from which significant numbers of asylum seekers have come."

Mr Fletcher said the countries would discuss the treatment of people returned.

Prime Minister Tony Abbott said: "They deserve to be in Iran, they belong in Iran if they're found not to be refugees".

Greens 'extremely concerned about any agreement'

The Greens raised the prospect of Iranians being killed if they are forced back after having asylum claims rejected.

"If Australia strikes a deal with Iran to deport vulnerable asylum seekers, families, children, men and women, I fear mistakes will be made, and these are life and death decisions for many of them," Greens senator Sarah Hanson-Young said.

"We know there are hundreds of Iranian asylum seeker families here in Australia as well as on Nauru and many others on Manus Island who are fearful of what will happen to them if they are returned to Iran.

"I am extremely concerned about any agreement Julie Bishop may strike with the Iranian Government to deport vulnerable asylum seekers back to that country.

"Julie Bishop and the Foreign Affairs department need to be crystal clear with the Australian public before they go — what conditions will they set down in terms of this negotiation?"

Ms Bishop's trip is a rare high-level visit from Australia to Iran.

Mr Fletcher said it is a significant trip and a range of other issues would also be discussed.

Ms Bishop is visiting Tehran as international nuclear talks with the country continue.

Iran is also playing a role fighting Islamic State.