Nick McKenzie wrote a series of reports for Nine newspapers in 2018 describing the former soldier as a war criminal, an allegation a judge later found was true on the balance of probabilities.
Roberts-Smith is pushing to reopen his appeal against the judge's finding, which he said involved a miscarriage of justice because McKenzie unlawfully obtained details about the former soldier's legal strategy.
McKenzie told the Federal Court on Thursday that he had "on occasion" used deceptive methods to obtain information in his role as a journalist if the matter was in the public interest.
He escaped conviction for illegally accessing restricted information after he and his colleagues gained unauthorised access to the Australian Labor Party database in 2010.
Under cross examination by Roberts-Smith's lawyer Arthur Moses SC, McKenzie accepted his methods had been unlawful but didn't agree he had crossed a line as a journalist.
"I try to act within the law. There (are) occasion(s) where as a journalist there can be things that you might do that conflict with the law," the journalist said.
"There are situations where it's our job to find information that has been hidden."
McKenzie received privileged information from Roberts-Smith's ex-wife Emma Roberts and used it to shape his defence to the defamation suit, the former elite soldier alleges.
In a taped call between the journalist and Roberts-Smith's ex-lover - known as Person 17 - he explains Ms Roberts and her friend Danielle Scott had been "actively briefing us on his legal strategy".
"I shouldn't tell you. I've just breached my f***ing ethics in doing that, like this has put me in a s*** position now," McKenzie said.
He maintains that he didn't know any of the information he received was privileged and says he passed on relevant communications to his lawyers.
The journalist said there was "at times desperation" and "intense anxiety" throughout the seven years of Roberts-Smith's defamation proceedings and subsequent appeal.
He had continued to investigate and search for evidence that would assist him and Nine in winning the case, the court was told.
"Throughout the whole proceeding, I was really anxious to prove that Ben Roberts-Smith was a war criminal and we had to find evidence to do that," McKenzie said.
He will continue to be quizzed on his journalistic methods and ethics when he returns to the stand on Friday.
The call that sparked the renewed bid to clear Roberts-Smith's name came after his appeal concluded in early 2024 but Mr Moses argues the matter should be considered before the judges hand down their decision.
If his appeal is unsuccessful, only the High Court could overturn the war criminal finding.
Roberts-Smith rose to prominence in 2011 after he was awarded Australia's highest military honour, the Victoria Cross, for single-handedly taking out machine-gun posts to protect pinned-down colleagues in Afghanistan.
His reputation, however, was tarnished in 2018 after McKenzie's explosive reports alleging the special forces veteran was complicit in the murder of four unarmed men during his deployment in Afghanistan.
In June 2023, Justice Anthony Besanko found the reports had been proven on the balance of probabilities - a lower standard than in a criminal proceeding.
