To make things even better for the Grove-based team, Kazuki Nakajima went second quickest in the dying seconds of the session.
What's more, both cars set their times on the harder of the two new slick compounds Bridgestone has brought to Australia.
Kimi Raikkonen was third fastest for Ferrari. The 2007 world champion dominated the top slot for much of the 90 minutes, with most of his opposition, as predicted, coming from the two Brawns of Jenson Button and Rubens Barrichello, until the Williams team ruined it for the team late on.
It was a quiet start to F1 2009, with almost no running at all in the first 30 minutes of opening practice, with the obvious exception of installation laps and some acclimatisation running for this year's only rookie Sebastien Buemi.
Perhaps the new rules, which insist that drivers use just eight motors through the 17 events, including Fridays, have the teams thinking about engine conservation even at the first race of the season.
Raikkonen was the first serious runner to hit the track at the half-hour mark and the Finn consistently lowered his lap time to a 1:28.458.
Surprisingly perhaps, it was then Heikki Kovalainen who took the top spot after 50 minutes with a lap of 1:27.743. The Finn having had more experience of the already-updated McLaren MP4-24 at the recent Jerez test than world champion team-mate Lewis Hamilton.
Kovalainen was almost immediately replaced at the top of the times by Barrichello, before Raikkonen moved ahead again when he bolted on a set of green-rimmed super-soft tyres to set a blinding 1:26.750.
Then, having hovered around the top six for most of the session, Rosberg settled the matter with that extraordinary lap.
In the end, the fastest of the much-vaunted Brawn GPs was Barrichello in fourth, despite a couple of trips across the grass. Kovalainen ended up a competitive fifth ahead of Button in sixth.
Felipe Massa was seventh, another to have a hairy moment under braking for Turn 14. Timo Glock's Toyota, Adrian Sutil's Force India and Fernando Alonso completed the top ten.
Hamilton, 16th overall, was one of several high profile drivers to run-off track during the session.
Other incidents included a spin for Nelson Piquet at Turn 1 and a puncture for Nakajima, while Sebastian Vettel stopped early-on on-track with a hydraulic problem on his Red Bull.
Pos Driver Team Time Laps
1. Rosberg Williams-Toyota (B) 1:26.687 19
2. Nakajima Williams-Toyota (B) 1:26.736 + 0.049 21
3. Raikkonen Ferrari (B) 1:26.750 + 0.063 24
4. Barrichello Brawn-Mercedes (B) 1:27.226 + 0.539 21
5. Kovalainen McLaren-Mercedes (B) 1:27.453 + 0.766 15
6. Button Brawn-Mercedes (B) 1:27.467 + 0.780 12
7. Massa Ferrari (B) 1:27.642 + 0.955 24
8. Glock Toyota (B) 1:27.710 + 1.023 24
9. Sutil Force India-Mercedes (B) 1:27.993 + 1.306 20
10. Alonso Renault (B) 1:28.123 + 1.436 16
11. Heidfeld BMW Sauber (B) 1:28.137 + 1.450 20
12. Trulli Toyota (B) 1:28.142 + 1.455 21
13. Kubica BMW Sauber (B) 1:28.511 + 1.824 22
14. Fisichella Force India-Mercedes (B) 1:28.603 + 1.916 16
15. Buemi Toro Rosso-Ferrari (B) 1:28.785 + 2.098 27
16. Hamilton McLaren-Mercedes (B) 1:29.042 + 2.355 18
17. Webber Red Bull-Renault (B) 1:29.081 + 2.394 7
18. Piquet Renault (B) 1:29.461 + 2.774 25
19. Bourdais Toro Rosso-Ferrari (B) 1:29.499 + 2.812 21
20. Vettel Red Bull-Renault (B) 1:32.784 + 6.097 4