In Senate estimates hearings on Thursday, Labor senator Penny Wong asked Department of Foreign Affairs and Trade officials to explain how much the Liberal Party elder and former immigration minister was being paid for serving as the government's special envoy for human rights.

After 42 years in federal Parliament, Mr Ruddock is entitled to an annual pension of about $215,000 per year, or a lump sum payment of about $2.15 million plus $107,500 per year.

He is also being paid for his new departmental contract, a per diem share of between $210,482 and $241,715, in line with a senior public service SES band 2 role. Mr Ruddock is only paid for days worked.

Former members of Parliament appointed to government jobs usually have pension payments adjusted to avoid double-dipping.

DFAT's multilateral policy division boss Lachlan Strahan told the hearing Mr Ruddock had sought legal advice about how the contract for his new envoy role would impact his pension entitlement.

"Legal advice was sought on that matter and the legal advice that we were given is that because he bears the title of envoy, it doesn't have an impact on his pension," Dr Strahan said.

"I gather it's not linked to the title - it's the type of title, I understand. He actually sought advice off his own bat about his pension."

Senator Wong expressed surprise at that revelation: "He gets double pay? That's very convenient, because he has a title?"

Prime Minister Malcolm Turnbull named Mr Ruddock to the envoy role in February, citing the Howard government minister's commitment to human rights.

His pay is the same as Australia's ambassador for women and girls, former Democrats leader Natasha Stott-Despoja.

Mr Ruddock said he had not been paid for work conducted before the federal election.

"The written advice from the Department of Finance is that if it is under a contract with the department - presumably it's happened before - it would have a very marginal impact on my pension, even if it were an office of profit under the Crown," he said.

"It's a daily rate and the impact on the pension is marginal."

Mr Ruddock has travelled overseas five times since taking the role, including trips to Switzerland, Norway, Guyana and various African countries, promoting Australia's candidacy for the United Nations Human Rights Council.

He is due to travel to Africa again next month and his term will end as voting for the council begins in October 2017.