Valtteri Bottas says it is difficult to judge where Williams are in the pecking order, saying they won't know until Australia.
The rookie racer and his team spent the last four days lapping the Circuit de Catalunya in the second of the three pre-season tests.
Williams put in some solid results with Pastor Maldonado's fastest time a 1:22.675 while Bottas clocked a 1:22.826. The week's best belonged to Sergio Perez with a 1:21.848.
"It's difficult to judge, but for sure Red Bull, McLaren and Lotus are strong," Bottas said.
"That's for sure, but maybe in the next test we'll see a little bit more, but finally in qualifying in Melbourne."
He added: "We're trying, that's our aim; to get higher."
Williams' programme on the final day at Barcelona was disrupted by rain, resulting in the team practicing pit stops throughout the day.
But despite losing a day's worth of lap, Bottas reckons his team's pre-season programme is on track.
"I think with everything we're really on the plan. Of course this was not planned, we missed some laps because of the rain but that's the same for everyone now so we're on the plan and we'll continue to push in all areas at the next test.
"There's some balance things in the car that we can still improve, especially with the little bit less fuel than high fuel.
"So we can still improve the car quite a bit and we are aware of the problems we have, they're all fixable so it's looking good."
Any reproduction, publication or redistribution of this material without the written agreement of 365 Media Group is strictly forbidden.