Valtteri Bottas finished the final day of pre-season testing on top and then declared Williams will take a "very competitive" car to Melbourne.
The Finn posted the fastest time of 1:23.063 in the morning session and he completed 89 laps before the Grove squad decided to end their running early on day four at the Circuit de Catalunya.
Williams have been solid throughout winter testing and many believe they will start the season as second favourites behind Mercedes, and it is hard to disagree going by Sunday's result.
"It was a really good day and a great way to finish the test. One of the main points we have learnt is that the FW37 is very reliable," he said. "The car we are taking to Melbourne is very competitive and should make for an interesting start to the season."
Bottas and team-mate Felipe Massa completed 384 laps between them over the four days in Barcelona.
"We have made the most out of our time testing and I need to say a really big thanks to the team, not only trackside but also everyone back at the factory as they have produced a really quick and reliable car," he added.