Nico Rosberg completed a clean sweep of fastest practice times for Williams in the final session at Melbourne
The German beat Toyota's Jarno Trulli to the top spot by just 0.003 seconds with a last-gasp time, while Jenson Button took third for Brawn GP.
Felipe Massa put his Ferrari in fourth place, but his team-mate Kimi Raikkonen managed only five laps before having to park at the far side of the circuit with a hydraulic problem, leaving him slowest of all.
Kazuki Nakajima (Williams) and Rubens Barrichello (Brawn) completed the top six.
The Brawn GP cars, freshly bedecked in Virgin sponsorship logos, had been quickest for much of the session.
Button set the first representative time with a 1:27.863 after 23 minutes, before Williams briefly reprised its Friday form as Nakajima and Rosberg took a one-two.
They were swiftly usurped by Barrichello, with the two Brawns and two Williams covered by just 0.134s as the session neared its halfway point.
Red Bull's Sebastian Vettel then raised the bar by lapping half a second clear of the rest of the field, but on their next runs the Brawns returned to the front, with Button producing a 1:26.691 to beat Vettel by 0.4s, and Barrichello then going a further 0.2s quicker on the softer tyres.
The rest of the field opted for soft rubber shortly afterwards, and with 12 minutes to go Timo Glock used the green-striped Bridgestones to put Toyota on top by 0.079s over Barrichello.
Button and Barrichello restored Brawn GP's superiority a few minutes later - only for first Trulli, then Rosberg to leap ahead with their final laps.
Trulli's 1:25.811 lap was the fastest of the weekend so far at that point, and although Button closed in, he could not beat the time. Rosberg could, however, lapping in 1:25.808 in the penultimate minute of the session.
Outside the top six, Mark Webber took seventh for Red Bull, as Glock slipped back to eighth position ahead of the two BMWs.
Raikkonen's failure was the most significant incident of the session, which saw various minor excursions - particularly at the tricky braking area for Turn 1. Nakajima and both Red Bulls visited the grass here, with Vettel slowing and complaining of brake problems afterwards.
McLaren showed better form than on Friday, with both Lewis Hamilton and Heikki Kovalainen featuring in the top five at times. They eventually fell to 11th and 12th, with Kovalainen also having a quick spin at Turn 4.
Renault also had a difficult session. Fernando Alonso was only 17th, while Nelson Piquet had several minor trips off the road and beat only Raikkonen on the timesheets.
Pos Driver Team Time Laps 1. Rosberg Williams-Toyota (B) 1:25.808 21 2. Trulli Toyota (B) 1:25.811 + 0.003 19 3. Button Brawn GP-Mercedes (B) 1:25.981 + 0.173 20 4. Massa Ferrari (B) 1:26.020 + 0.212 17 5. Nakajima Williams-Toyota (B) 1:26.078 + 0.270 18 6. Barrichello Brawn GP-Mercedes (B) 1:26.348 + 0.540 19 7. Webber Red Bull-Renault (B) 1:26.355 + 0.547 16 8. Glock Toyota (B) 1:26.410 + 0.602 25 9. Kubica BMW Sauber (B) 1:26.514 + 0.706 18 10. Heidfeld BMW Sauber (B) 1:26.555 + 0.747 19 11. Kovalainen McLaren-Mercedes (B) 1:26.652 + 0.844 13 12. Hamilton McLaren-Mercedes (B) 1:26.714 + 0.906 18 13. Vettel Red Bull-Renault (B) 1:27.009 + 1.201 12 14. Sutil Force India-Mercedes (B) 1:27.062 + 1.254 12 15. Buemi Toro Rosso-Ferrari (B) 1:27.152 + 1.344 16 16. Bourdais Toro Rosso-Ferrari (B) 1:27.192 + 1.384 17 17. Alonso Renault (B) 1:27.357 + 1.549 18 18. Fisichella Force India-Mercedes (B) 1:27.492 + 1.684 20 19. Piquet Renault (B) 1:27.739 + 1.931 22 20. Raikkonen Ferrari (B) 1:28.801 + 2.993 5 All timing unofficial
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