He spent the morning practising pit stops and aerodynamic assessments before recording his best time of one minute 25.837 seconds after an engine change - 4.5 seconds behind pacesetter Michael Schumacher for Mercedes.
Ferrari's Fernando Alonso trailed the German by 0.346 seconds, while Red Bull's Sebastian Vettel finished 1.665 seconds off the pace.
Although Button said he was disappointed not to get more miles under his belt in testing, he is looking forward to the season's first race.
"We had an engine change over lunch which took a little bit more time than we thought so I haven't got a lot of running done really," he said.
"But I think after Tuesday we were quite positive with the direction we've taken with the car and I think there is more to come in Australia."
I don't think you should ever write off a top team, especially one that's won so many championships and fought for so many championships like McLaren
Jenson Button
"It's a circuit I love and I've had great experiences there," said Button. "I'm looking forward to going back and hopefully we'll find a bit more performance and our reliability will be good."
Button is equally bullish about McLaren's chances of finding the pace and reliability to contest the championship over a season.
"I don't think you should ever write off a top team, especially one that's won so many championships and fought for so many championships like McLaren," he said.
"I agree that it's been a tough winter and we haven't done the mileage that we'd have liked and we haven't been able to get the performance out of this car that we expect to.
"But it doesn't mean that we're not going to be challenging this year. The first race is going to be difficult to really challenge the Ferraris and the Red Bulls, but you never know."
Button is hoping his Japanese girlfriend Jessica Michibata, who was in her homeland during the
earthquake that hit the country
on Friday, will also be in Melbourne to cheer him on.
"She's OK, just a bit shaken," he said. "Hopefully I'm going to see her in a couple of days in Australia if they can get flights out of Japan and give her a big hug."
Button added: "It's horrific and very devastating. It's difficult to know what to do but my thoughts are with all the people in Japan, especially in Sendai."
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