The Renault driver beat Rubens Barrichello's time by less than a tenth of a second, as the Brawn GP team was not on top of the times for the first time in the last four sessions.
Alonso managed over 100 laps with the R29, enjoying a more productive day than on Sunday.
"I'm happy with the day as we did more than a 100 laps which was our target for my final day of winter testing," said Alonso. "This has been the first time we have run on a really hot track and the information we have from today will be very useful for the first few races, which are usually very hot.
"I'm also a bit sad that this is the final test of the year with my test team, but I will see them at the factory soon. I have to say thanks for the amazing job they have done this winter because we are really well prepared for the new season. We can go to Australia knowing that we have maximised the time we had available."
Brawn GP used both Barrichello and Jenson Button today, the duo finishing second and third quickest. The Briton, however, managed just 12 laps, his car coming to a stop at around 3pm local time.
Lewis Hamilton kicked off McLaren's final test before the Australian Grand Prix, the world champion finishing in fourth place over a second behind Alonso.
Nico Rosberg was the fifth runner on track, having replaced Williams' test driver Nico Hulkenberg at the wheel of the FW31.
More to follow.
Pos Driver Team Time Laps
1. Alonso Renault (B) 1:18.343 103
2. Barrichello Brawn-Mercedes (B) 1:18.398 62
3. Button Brawn-Mercedes (B) 1:18.892 12
4. Hamilton McLaren-Mercedes (B) 1:19.513 85
5. Rosberg Williams-Toyota (B) 1:19.783 123