Australian Federal Police Detective Superintendent Scott Moller is the first police officer to appear before an independent inquiry into how the justice system handled Ms Higgins' allegation.
Supt Moller was the lead officer who investigated Ms Higgins' allegation Bruce Lehrmann, a former colleague, raped her after a night out in 2019, inside the Parliament House office of then coalition minister Linda Reynolds.
Mr Lerhmann denies the allegation.
The senior officer will be questioned by Mark Tedeschi, representing the Director of Public Prosecutions Shane Drumgold, after he told the inquiry he was concerned the prosecutor had tried to "collect evidence" to criticise police during the case.
"I had that feeling very early on to be honest. It had caused me quite a lot of concern for myself and my investigation team," Supt Moller said on Monday.
"So much so that we were continually second-guessing ourselves about what we were doing and how we were doing it."
Supt Moller said because of the prosecutor's "scrutiny", he wanted to make sure police were "doing everything perfectly".
"I didn't want to miss anything because I was continually concerned about the director and what I thought was collecting information about us not doing the right thing," he said.
Mr Drumgold previously told the inquiry he believed police had a "passion" for the prosecution to fail and criticised their decision to interview Ms Higgins for a second time, which was unusual in a sexual assault case.
But Supt Moller stood by the decision and said officers had an obligation to investigate inconsistencies in a complainant's account.
Officers were concerned by some of the evidence given to them by Ms Higgins and wanted to clarify this through a second interview.
"It's incumbent upon us as investigators to do the most complete and comprehensive job we can and that's what we try and do," Supt Moller said.
"We've got to support and protect the victim, but we've also got an obligation to collect the evidence ... so it's an ongoing battle for us."
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