Mel, 60, and equestrian-vaulter-turned-scriptwriter Rosalind, 26, met in early 2015, when she applied for a job at his production company. The baby will be the first child for the couple.
Mel is already father to seven children with ex-wife Robyn Moore - one daughter, named Hannah, and six sons, Edward, Christian, William, Louis, Milo and Thomas. He also has another daughter, six-year-old Lucia, from a previous relationship with Oksana Grigorieva.
PEOPLE magazine broke the news of Rosalind's pregnancy, with a source close to the couple telling the title: 'Mel and Rose are so excited about the baby'.
'Mel loves being a dad and he and Rose can’t wait to be parents together. The last two years have been some of his happiest years he’s ever had.'
Mel has kept a low profile over the last decade, keeping his personal life very private, but another insider recently revealed to the mag: 'She [Rosalind] is great for him. She has her own life, but also seems to enjoy Mel’s … They are very much on the same page.'
However, being part of an age-gap relationship in the public eye isn't always easy, as Rosalind's close friend Devon Maitozo revealed in an interview with the Daily Mail earlier this year.
Explaining that her friend had been struggling with 'negative attention' resulting from the relationship, Devon said, 'There's been a lot of not very nice media that's been written about her. Sifting through it has not been very pleasant at all. Sensationalising her because of her relationships is not what she's looking for.'
'She would probably enjoy it if it was about her and her career instead of her being the girlfriend of a celebrity, but it's not and so she'd like to keep her life private. She's just a young girl and wants to get on with her life without all the attention.'