Mr Martin admitted he lied to police when he initially told them he had woken briefly to some noises before going back to sleep,telling the court that he had withheld the truth for multiple reasons.
"The first one obviously being that I didn't want to be a part of what I had witnessed,which now has worked out ... worse,"Mr Martin told the court under cross-examination by Mr de Belin's barrister David Campbell,SC.
"The second reason,I also knew that Mr de Belin had a partner,and I didn't want to be the responsible one for telling that he was cheating on her."
Asked if there was a third reason,Mr Martin said he also had the distinct impression that police already knew someone else had been inside the apartment,and there were security cameras outside they would probably check.
"[I thought] they're going to find out from someone else,not me,"he said.
Mr de Belin,29,and Mr Sinclair,23,are on trial before Wollongong Judge Andrew Haesler after both pleaded not guilty to five charges each of aggravated sexual assault,claiming the encounter with the then-19-year-old woman after a night of partying was consensual.