Veteran Fernando Alonso said Thursday he was brimming with confidence heading into the Australian Grand Prix, but was cautious about whether Aston Martin could maintain their red-hot early season form.

The Spanish star, who won in Melbourne 17 years ago piloting a Renault, has secured stirring podiums at both races so far this year, with only the dominant Red Bulls of Max Verstappen and Sergio Perez quicker.

For the 41-year-old two-time world champion, the most experienced Formula One driver in history, it has been a welcome change of fortune after a series of fruitless years.

"At the moment, we are extremely happy with the performance of the car obviously," he said.

"We were better than expected, especially in Jeddah -- I think it was a nice surprise, so we arrive here with a good level of confidence."

His third place in Saudi Arabia a fortnight ago was Alonso's 100th podium and put him in rarified company as only the sixth driver to achieve the feat.

Alonso, who is still searching for a first win since Spain 10 years ago, admitted it was an honour, but said personal milestones were not on his mind.

"It feels good, it feels nice to reach that number, but obviously when you are in the championship you are not really concentrating on that, you are just thinking of the next weekend," he said.

While he heads into Sunday's race with his spirits high in a car that has made giant strides, Alonso said it was too early to say whether they could sustain their competitive edge throughout the season.

"Not yet. I think we have to wait and see," he said.

"Also I think very soon, from Baku or Imola, we will see the teams changing a few parts on the cars and maybe that changes also how competitive you feel."