The Queensland senator faced the High Court's Court of Disputed Returns in Brisbane on Thursday, after it was revealed he received a letter confirming he renounced his ties to Britain five months after the 2016 federal election.

Section 44 of the constitution prohibits dual citizens from serving in the Federal Parliament.

Senator Roberts was born in India in 1955 to an Australian mother and Welsh father, and became an Australian citizen when he was 19.

His case is likely to hinge on whether sending the emails about his citizenship constituted taking "reasonable steps" to renounce his British citizenship.

The High Court heard Senator Roberts sent emails - on May 1 and June 6, 2016 - regarding his citizenship to email addresses that did not exist at the UK consulate and British High Commission, and signed the AEC forms to nominate on June 3.

But he did not seek legal advice over his citizenship status.

Senator Roberts rejected the suggestion that sending an email with the subject line "Am I still a British citizen?" meant he thought he was British at some point.

More than half-a-dozen MPs have been caught up in the dual-citizenship saga, including Deputy Prime Minister Barnaby Joyce.

Senator Roberts' barrister Robert Newlinds accepted his client would have been a British citizen when he lodged his nomination form to become a candidate in the federal election in July 2016.

"We don't contest that he was a British citizen at that date," Mr Newlinds said.

"He didn't know that, or to use a more helpful word, he didn't believe he was and never has."

However, Senator Roberts said the High Court hearing was the first time he had heard his lawyers believed he was British.

Senior counsel Stephen Lloyd, appearing as a friend of the court, said two of the email addresses used by the One Nation Senator were decommissioned by the British government in 2010 and Senator Roberts did not receive a response.

Senator Roberts said he found the email addresses by searching online, and did not think anything was wrong, despite not receiving a response from the British authorities.

"I didn't get an error message," he said.

"I can only go off what I found on the internet and believe to be entirely true and valid."

Mr Lloyd said one of the email addresses appeared to be in use in mid-2016, but others, including one with a "UK.Sydney" suffix, were not correct.

The email from June said: "With this email, I renounce any British citizenship, should it exist", the court was told.

Mr Newlinds admitted Senator Roberts could have done more to find out whether he held dual citizenship and renounce it.

Senator Roberts insisted he always believed he was Australian, despite coming to Australia for the first time in 1962, as that was the way he was "raised", and he had travelled on his mother's passport.

He said when he nominated as a candidate, he was sure he was Australian.

"[But] on reflection, I realise there's a possibility of being Indian, and a possibility of being British," he said.

However, at another point during the hearing, Senator Roberts again said he believed he "never had citizenship of another country".

"I would be happy to say that because no one has ever shown me I had citizenship of another country. But I believe there are various other requirements so I can't rule it out altogether," Senator Roberts said.

"I believed I was always Australian and only Australian."

Senator Roberts said he believed he was an Australian citizen because of his father's attention to detail, memory and sense of humour and the way he was brought up.

He said his dad loved to "rib him" whenever he contradicted himself.

"He never contradicted me on my statement that I was an Australian ... Not once did he do that, if he'd known that, he would have," he said.

However, his own barrister, Mr Newlinds, said it was inevitable Justice Patrick Keane would find that by the time Senator Roberts wrote the emails, he would have understood at some level he was probably British up to the time of Australian naturalisation - but he believed he was solely Australian when he nominated for the election.

"He's talked himself into a different belief about that," he said.

When he was 19 years old, Senator Roberts signed a one-sentence form to become an Australian citizen, which he said his 16-year-old sister Barbara Roberts filled out.

He said he did not read the forms, did not remember signing it, but would have done it because his father told him to.

"I was 19 and more keen on football than on filling out forms," he said.

Senator Roberts said he asked his sister about his citizenship and she replied: "Stateless".

Ms Roberts, who gave evidence, said she did not remember filling out the forms, but speculated her mother told her what to write on the forms because some of her own information was wrong, such as her eye and hair colour.

The court was also told there was an affidavit from an expert that said he would have been an Indian citizen at birth but he would have lost it upon becoming an Australian at age 19.

Legal experts from London told the court via audio link, the renunciation must be made to the Home Office, but admitted UK nationality laws were very complicated.

British barrister Laurie Fransman said a fee must accompany an application to make it valid, whereas the other expert disagreed with the interpretation.

Justice Keane hopes to deliver his findings of the facts on Friday, but further examination of the case will continue at a later date.