"English Patient" star Ralph Fiennes is accused of having a "mile-high" fling with a flight attendant, the Australian Sunday Telegraph reports.
Two other crew members saw Robertson, 38, chatting with Fiennes, 44, and they waited for them to come out of the bathroom.
They then reported the incident to a senior crew member.
In a statement lodged with her employer, Robertson denied the allegations.
"While conversing with Mr. Fiennes during my break, I expressed a need to go to the toilet,'' Ms Robertson said in her statement.
"I went to the nearby toilet and entered it, he followed me and entered the same toilet.
"I explained to him that this was inappropriate and asked him to leave. Mr. Fiennes became amorous towards me and, after a short period of time, I convinced him to leave the toilet, which he did," she said.
Robertson was reportedly in hiding this weekend after being grounded without pay.
Fiennes was flying from Australia to India as a Unicef ambassador to promote awareness of HIV and safe sex at the time.