Growing up in the Sydney Croatian community, Kalac began his professional career rising through the ranks at Sydney United, competing in the Australian National Soccer League. At the age of sixteen, he was included in the first team alongside first-choice keeper Tony Franken, playing the first eleven games of the 1989–90 NSL season in Franken's absence, before returning to the substitute bench – a place occupied throughout the 1990–91 season as a stand-out Franken remained fit and was first choice all season.