He says some footballers are happy to run the risk of being tested while on a high - natural or otherwise - after completing a gruelling season in the ultimate fashion.
"Society is society and back when I was playing you knew it would get out of your system in 48 hours," he said. "That's what all the breaking your back for all year is: to have just maybe one night where you'd party all night and let your hair down."
A former New Zealand junior decathlon champion who once trained under Daley Thompson in Britain, Donnelly is adamant he never engaged in recreational drugs - but he saw it first hand.
"I was too focused, obsessed, growing up on being an athlete. I wanted to go to the Olympics," he said. "The first time I was involved in a grand final, I was a bit overawed. I was only 21 and had come over from New Zealand. So then after the grand final, to see all the cocaine use was a bit of a surprise."
Donnelly, a nuggety powerhouse and one of coach Brian Smith's trailblazing metre-eating wingers, also cast doubt on the consistency of drug testing.
"There's no way that drug testing is random," he said. "I was targeted because of my size to height but never failed any of the six or so tests I ever had. But I know teammates who were never tested.
"Ben Barba's 12-match suspension is what he deserves but I think it (party drug use) is rife."
Cronulla stars made no secret on social media of the fact their premiership party lasted for days, posting numerous celebratory images but Sharks chairman Lyle Gorman said he'd been assured by the NRL that no other players tested positive to illicit drugs.