The lawyer for One Nation senator Malcolm Roberts has said his client's credibility may have been affected by evidence given during a lengthy cross-examination at the High Court on Thursday.

Senator Roberts admitted to signing a form which said he was a British citizen, but says he did so without reading the document.

Key points: Senator Roberts sent two emails to inactive addresses during attempts to check his citizenship status

Senator Roberts sent two emails to inactive addresses during attempts to check his citizenship status Senator Roberts conceded he did not read his citizenship form, which his sister filled out for him

Senator Roberts conceded he did not read his citizenship form, which his sister filled out for him He said it remained unclear whether he still held his British citizenship

Senator Roberts' claims he did everything he could to renounce his British citizenship before the 2016 federal election were being tested at the hearing in Brisbane on Thursday.

The constitution bans people with dual citizenship from being elected to Federal Parliament.

Despite Senator Roberts' lawyer Robert Newlinds accepting expert advice that his client had once been a British citizen, Senator Roberts refused to concede the issue.

"I'll have a conversation with my barrister on that," Senator Roberts said, when asked whether he now accepted he was once British.

Later, Senator Roberts said when he nominated for Parliament he did so with a firm belief he was Australian only, but that there was a "very small possibility" that he might have also held British or Indian citizenship.

Mr Newlinds said it was "inevitable" the court would find Senator Roberts had some knowledge that he was British in the lead-up to him becoming an Australian citizen in 1974.

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

"He'll have to deal with that insofar it affects his credit for other things he says."

Questions were first raised as to whether Senator Roberts held dual citizenship because he was born in India to a Welsh father and Australian mother.

Under questioning from counsel assisting the court Stephen Lloyd, Senator Roberts said his father had given him an Australian citizenship form in 1974 which his then-16-year-old sister filled out and he signed without reading.

The form contained a line which said Senator Roberts held UK and Colonies citizenship.

"If my father had said 'here, sign this' I would have," he said.

Senator Roberts conceded he would have realised he was a British citizen if he had read the form.

He said from birth, he was always raised to believe he was Australian only.

This was based on the fact he travelled on his mother's passport as a boy and that his father never mentioned formal links to Britain.

"He would have ribbed me for sure if he knew I was British. He would have let me know in no uncertain terms," Senator Roberts said.

"All my correspondence with the UK prior and after the nomination [for Parliament], no-one from the United Kingdom has said I am British. No-one."

The court heard Senator Roberts was automatically granted Indian citizenship by birth, but that was renounced when he became an Australian citizen when he was 19 years old in 1974.

'Am I still a British citizen' email sent to wrong address

Mr Lloyd said Senator Roberts sent emails to a non-existent email address and a decommissioned account that had not been used for six years while trying to make inquiries about the potential of him holding British citizenship prior to the 2016 election.

He said the emails to British authorities were sent on May 1 and June 6, 2016.

His first email, titled "Am I still a British citizen", went to an address ending with the domain ".uksydney", Mr Lloyd said.

Mr Lloyd asked Mr Roberts: "Have you ever seen any email address that ends with anything like '.uksydney'?"

"No," Mr Roberts replied.

Mr Roberts said he found the email address online but could not remember where.

The next email, complaining that nobody had got back to him, went to a decommissioned account which had been inactive for six years, Mr Lloyd said.

"None of these emails led to any responses," Mr Lloyd said.

He said Senator Roberts sent a third email with inquiries and that one worked.

Mr Roberts repeatedly denied he knew he was British when sending the emails, but conceded that he felt there was "a very small possibility".

Mr Lloyd said other evidence included documents Mr Roberts received while he was preparing to nominate himself as a senator, which provided tips on what candidates could do if they had doubts about eligibility.

Findings could be delivered on Friday

Thursday's hearing served primarily as a fact-finding mission, and Justice Patrick Keane said he hoped to deliver findings on Friday.

But the question of whether Senator Roberts took all reasonable steps to renounce his citizenship is likely to be answered by the full bench of the High Court in Canberra, which will begin hearing submissions on his case and several others next month.

Senator Roberts is one of seven federal politicians embroiled in the dual citizenship crisis gripping Canberra, including Deputy Prime Minister Barnaby Joyce.

The Queenslander has long protested against questions about his citizenship, suggesting it was another attack against One Nation.

When he asked the High Court to consider his citizenship status, Senator Roberts further confused matters by repeatedly refusing to say when he filled in the form renouncing his British citizenship and paid the associated fee.

Last week, his lawyers also told the court they intended to challenge expert legal advice the Commonwealth had received from a British barrister about Senator Roberts.

His lawyers also suggested they would argue Senator Roberts did not have to have formally filled out the renunciation form to have kickstarted the process.

Their argument was that his attempts to email the British Home Office to notify them he intended to renounce his citizenship was enough.