Schwantz pushed his Suzuki around the purpose-built circuit at Eastern Creek in 1min 31.243sec, followed by Rainey, the three-times world champion, in 1:31.679.
Australia's Daryl Beattie was third fastest on a Honda, but the surprise performance of the first day's practice came from the young American, Doug Chandler, who was fourth on a Cagiva in 1:32.187.
Michael Doohan, who won this race last year, came through his injury test with flying colours. The 27-year-old Australian broke a bone in his left wrist while testing his Honda in Malaysia earlier this month. Initially tentative, Doohan grew in confidence and emerged ninth fastest in 1:32.895.
'The wrist's a little sore but it wasn't too bad,' said Doohan, adding that the number of left-hand bends caused him the greatest difficulty.
Doohan has suffered from a series of injuries but nevertheless managed to build up a 53-point lead in the championship last year. However, he broke his right leg in a crash before the Dutch Grand Prix and ultimately finished four points behind Rainey, settling for the runner-up position for the second successive year.
Heavy rain late in the qualifying session left the tight Eastern Creek track in a treacherous condition and led to Cees Doorakkers, of the Netherlands, losing control of his Yamaha. Fortunately, neither rider nor machine was seriously damaged. The second and final qualifying session takes place today.
In the 250cc class, the Italian Massimiliano Biaggi recorded the fastest time on his Honda - 1min 32.898sec - ahead of the Suzuki of the American John Kocinski, making his return to the class after two years riding with the Roberts 500cc team.
AUSTRALIAN GRAND PRIX (Eastern Creek Sydney): Leading qualifying times: 500cc: 1 K Schwantz (US) Suzuki 1min 31.243sec; 2 W Rainey (US) Yamaha 1:31.679; 3 Daryl Beattie (Aus) Honda 1:31.929; 4 D Chandler (US) Cagiva 1:32.187; 5 A Criville (Sp) Honda 1:32.433; 6 A Barros (Bra) Suzuki 1:32.619; 7 S Itoh (Japan) Honda 1:32.637; 8 P Goddard (Aus) Suzuki 1:32.651; 9 M Doohan (Aus) Honda 1:32.895; 10 M Mladin (Aus) Cagiva 1:33.034.
250 cc: 1 M Biaggi (It) Honda 1:32.898; 2 J Kocinski (US) Suzuki 1:33.201; 3 L Capirossi (It) Honda 1:33.454; 4 T Harada (Japan) Yamaha 1:33.754; 5 T Okada (Japan) Honda 1:33.782.
125cc: 1 R Waldmann (Ger) Aprilia 1:41.371; 2 L Bodelier (Neth) Honda 1:41.475; 3 D Raudies (Ger) Honda 1:41.809; 4 H Aoki (Japan) Honda 1:42.367; 5 G Palmieri (Swit) Aprilia 1:42.499sec.
Full 1993 calendar (125cc, 250cc and 500cc): 4 April: Malaysia (Shah Alam); 18 April: Japan (Suzuka); 2 May: Spain (Jerez); 16 May: Austria (Salzburg); 30 May (to be announced); 13 June: Germany (Hockenheim); 27 June: Netherlands (Assen); 4 July: Europe (Catalunya, Barcelona); 18 July: San Marino (Mugello, Italy); 1 August: Britain (Donington); 15 August: Czech Rep (Brno); 5 September: Italy (Misano); 12 Sept: United States (Laguna Seca); 3 Oct: South Africa (Kyalami).Reuse content