Rosberg also tops final Australia practice

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