Senator Roberts is in court facing cross-examination over whether he renounced his British citizenship before contesting a seat in the Australian parliament.
Just before he signed his Senate nomination form on 8 June, he told the court, he allowed a small amount of doubt to spur him into checking his citizenship status with the United Kingdom authorities, sending emails to the consulate for clarification on his status.
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.
Roberts was born in India in 1955 to a Welsh father and Australian mother but has argued he was only ever Australian.
His case is likely to hinge on whether sending the emails about his citizenship constituted taking "reasonable steps" to renounce his British citizenship.
Last week, his lawyers also told the court they meant to challenge expert legal advice the Commonwealth had received from a British barrister about Senator Roberts.
"I would be happy to say that because no one has ever shown me I had citizenship of another country".
The email was sent to both valid and invalid email addresses. Senator Roberts said he never received a "bounce back" from any of the emails.
The court heard he then asked his sister Barbara Roberts what his nationality was and she told him they had been "stateless". (Senator Roberts admitting the form was written out in the hand-writing of his 16-year-old sister). Roberts was also backed by the authority of Sky TV's Paul Murray, who said the documents were "a million per cent correct" and "very clearly" proved Roberts was not a dual citizen.
Senator Roberts also revealed to the court his two children, now aged in their 20s, had become U.S. citizens by descent because American wife filled out an application form shortly after their respective births.
Less than a year later, in July 2017, Roberts's staff claimed "any renunciation" of what, until then had been a non-existent dual citizenship, was made in June 2016, well before the nomination date.
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 exclusively Australian when he nominated for the election.
On whether One Nation checked his nomination form: "No one did any vetting, that I'm aware of, with me".