The insider added: "It wasn't exactly a meeting of minds. Neither saw it as long-term, but there was chemistry there. Mel thought Zac had an amazing body."
MailOnline has contacted Mel and Zac's representatives for comment.
News of Mel's reported tryst with Zac comes as the star continues to be locked in a bitter custody battle with her ex-husband Stephen, with the star's ex-nanny recently deeming her an unfit mother.
On Saturday, ex-nanny Rusty Updegraff insisted he gave his testimony claiming she was neglecting her children due to a drink, drugs and sex addiction to help the Spice Girl.
He that her drinking was out of control, she was 'incapable' of caring for Madison and her 11-year-old sister Angel and her ferocious appetite for sex made her bed a slew of random men.
The 59-year-old has now claimed he didn't make the allegations for 'fame', but out of genuine concern for the singer and her children.
"Mel would drink so much at the house and pass out that if there was an emergency I dread to think what would happen," he told The Mirror.
"I am not doing this for fame, and I didn't have to do this, but I love those children and want the best for them."
