Bellamy (20) carded 69 and 77 to share the two-round lead with Brandon Hodgson (Otago) and Brent McEwan (St Clair) on 146.
It was tight at the top with Nick Borren (Otago), Matt Tautari (Southland) and Mark Brooks (Queenstown) tied one shot off the pace.
Gary Creedy (North Otago), the best of the masters (over-40) golfers, was a further shot back after two rounds of 74.
Abigael Crawford (St Clair), the only woman in the field, made her mark with an impressive 78 in the morning.
It was a family affair for the Bellamys as Ryan's younger brother, Jacob (16), carded 84 and 81 to claim handicap honours.
However, Ryan's win did not come without considerable drama as the sudden-death playoff involving the top trio took on a life of its own.
Bellamy, who was awarded the Ralph Hotere Memorial trophy last year as Port Chalmers' leading junior, set up his win with a three-shot lead at lunch after finishing 3-under on 69 in the morning.
He turned 1-over but notched four birdies on his homeward run.
He had three opportunities to seal the win but the first went begging when he carded three bogeys on his final nine holes to surrender the outright lead.
On the first hole of the playoff, he missed a short a short birdie putt to halve with McEwan while Hodgson bowed out after he failed to get up and down.
The remaining pair halved the next playoff hole with pars and, as darkness fell, they elected to play on rather than accept a tie.
McEwan's drive went up a tree and was eventually lost and he elected to return to the tee to play his third, only to hit Bellamy's trolley which incurs no penalty.
Bellamy made a two-putt par-4 which gave McEwan no chance of competing with his second ball.
The trial series continues with a 72-hole event at Queenstown this weekend.
