Born in Goulburn, New South Wales, Poidevin played rugby at St Patrick's College (now Trinity Catholic College) in New South Wales, and made the Australian Schoolboy side. Upon finishing school he played a season with the Goulburn Rugby Union Football Club and then, in 1978, he moved to Sydney to study at the University of New South Wales, from which he graduated in 1983 with a Bachelor of Science (Hons). He made his first grade debut with the university's rugby union team in 1978. In 1982 he moved clubs to Randwick, the famous Galloping Greens, home of the Ella brothers and many other Wallabies.

In March, Poidevin played in the World Sevens at Concord Oval. Australia was defeated by New Zealand 32-0 in the final. The final was the first time that Poidevin would oppose Wayne "Buck" Shelford, in what would be the beginning of a fierce rivalry between the two men. In For Love Not Money Poidevin remembered that:  It was a tremendously physical game and was marred by Glen Ella being elbowed in the head by Wayne Shelford. It was the first time I'd come up against this character and to say I didn't like his approach was putting it mildly. I was sickened by what he did to my Randwick clubmate and simply couldn't contain myself. Within a minute of his clobbering Glen I got into a stouch with him and we finished up rolling around on the ground in front of the packed main grandstand, not only in front of Premier Neville Wran but in front of a far more important person - my mother. While we were grappling I thought to myself 'we really shouldn't be doing this', but my blood was boiling after the Ella incident.  Poidevin then participated in the Hong Kong Sevens where Australia were knocked-out in the semi-final by the French Barbarians. He would later reflect that 'I thought my own play was diabolical. They scored a couple of easy tries early on through what I felt was my lax defence.' He further added that, 'I was pretty chopped up after that loss, particularly as I'd been very keen to make the final so that I could have another crack at the New Zealanders.'

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