Fiona Brown, Linda Reynolds' former chief of staff, will testify this week.
Ms Brown's doctors advised her not to give evidence due to mental health issues, but her lawyer told the court she was happy to contribute.
The court's live stream may be temporarily disabled while she is being questioned.
Ms Brown worked for Ms Reynolds when she took over the defense industry portfolio in early 2019.
Brittany Higgins and Bruce Lehrmann worked as consultants for Ms. Reynolds at the time.
Ms Higgins claims Ms Brown was the first person she disclosed her alleged rape to at a private meeting in the last week of March that year.
Mrs. Brown was the only person to take notes immediately after those meetings and was therefore once called “the most important person in the case.”
She previously told the court that Ms Higgins said Mr Lehrmann was on top of her, but she did not use the word “rape”.
Ms Brown has consistently denied failing to support Ms Higgins after she made her rape allegations.
She gave evidence at Lehrmann's criminal trial last year, but ran from the courtroom in tears.
Veteran television boss Peter Meakin will testify on Monday, along with The Project producer Chris Bendall and Queensland MP Sam O'Connor.
The case is expected to conclude this week, with final submissions expected Thursday or Friday.