Lewis Hamilton set the fastest time of testing so far at Barcelona on Saturday, and team-mate Rosberg kept Mercedes in front with an even quicker lap on the final morning.
Mercedes' pace has also impressed the rest of the pitlane, but Rosberg warned that the only thing the team should be confident of was its own progress.
Hamilton: Mercedes can win this year
"What I'm sure of is that definitely as a team we've progressed from last year," he said.
"I think we're in a much better position now. Communication, the structure and everything is working better.
"You need to be really, really careful about the pace.
"I know for a fact that other competitors haven't shown anything near what they're able to do. In Melbourne we'll learn more.
"Amongst other things, Red Bull is a big uncertainty."
Despite his caution over Mercedes' place in the pecking order, Rosberg said the W04 was undoubtedly better to drive than its predecessor.
"I think they've managed to get the balance quite nice, so it's felt good to drive from the first lap onwards this winter," he said.
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