And Senator Roberts' attempt to renounce his British citizenship via an email would not be effective, Justice Patrick Keane said in a judgement handed down late Friday afternoon.

Justice Keane delivered his findings after the Queensland senator faced the High Court, sitting as the Court of Disputed Returns, in Brisbane a day earlier.

Earlier this year, it was revealed Senator Roberts received a letter confirming he renounced his ties to Britain five months after the 2016 federal election, but Section 44 of the constitution prohibits dual citizens from serving in the 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 years old.

His case hinged on whether sending emails about his citizenship constituted taking "reasonable steps" to renounce his British citizenship.

Justice Keane said the only step arguably taken by Senator Roberts to renounce his foreign citizenship before his nomination was an email of June 6."

But British barristers Laurie Fransman and Adrian Berry agreed that email could not be effective as a renunciation because it was not sent to the appropriate authority, the Home Office in the UK.

"This email could not be effective as a renunciation of his UK citizenship because it did not contain a declaration of truth, and it was not accompanied by the prescribed fee," Justice Keane said.

Justice Keane found the email sent in June was "not apt to lead to a termination" of his British citizenship.

On Thursday, 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 in Brisbane and British High Commission after finding them on the internet, but he did not seek legal advice at the time.

The email addresses were either previously decommissioned or did not exist, although one appeared to be valid, although he never received a response.

On May 1, he emailed "bcabris2@britaus.net" and "bcabris1@britaus.net", which he believed were two email addresses at the UK Consulate in Brisbane, after he found them searching online, with the subject line: "Am I still a British citizen?"

In that email, he said: "Around 1973 my father and I and my brother and sister became Australian citizens."

"I am NOT seeking British citizenship. If I am still a UK citizen I need to renounce it for compliance with electoral legislation in Australia... If I am still a British citizen, how do I renounce it?"

Three days after submitting his nomination form, he wrote another email on June 6 to those same two email addresses and a third, "australia.enquiries@fco.gov.ukSydney", which he said belonged to the UK High Commission in Canberra, after he did not receive a reply.

"With this email I renounce any British citizenship should it exist. If I remained a British citizen after gaining Australian citizenship on 17th May 1974, please accept this email notice renouncing any remnant British citizenship, effective immediately," he wrote.

Despite those queries, Senator Roberts insisted he had always believed he was Australian during the court hearing.

Appearing relaxed with his fingers interlocked and often smiling during his hearing, Senator Roberts said he never had citizenship of another country: "I believed I was always Australian and only Australian."

But Justice Keane poured cold water on Senator Roberts' assertions in the court that he had always believed he was Australian, pointing out he had questioned whether he was "still" a British citizen via email.

He also said Senator Roberts did not suggest he renounced his British citizenship when he became naturalised as an Australian.

"The absence of any such suggestion highlights that there was no rational basis for the belief that he was always and only an Australian citizen," Justice Keane said.

He also said Senator Roberts appeared to equate feelings of Australian self-identification with citizenship.

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

When he was 19, Senator Roberts signed a one-sentence form to become an Australian citizen, but he denied any memory of it and said he would have signed whatever his father told him to.

But Justice Keane found Senator Roberts knew he did not become an Australian citizen until May 1974, when he signed the forms.

"I find that, as at the date of his nomination for the Senate, he knew that there was at least a real and substantial prospect that prior to May 1974 he had been and remained thereafter a citizen of the United Kingdom," Justice Keane said.

Senator Roberts asserted he was an Australian citizen by descent from his mother but had no answer on why he would not have similarly acquired British citizenship by descent from his father.

He once asked his sister about his citizenship and she replied: "Stateless".

Senator Roberts is due to face the full bench of the High Court next month, at which his eligibility to be a senator is expected to be decided.