Australia are eyeing a fairytale first home regatta win in two years but it's Spain atop the Sydney SailGP leaderboard after the first day of racing on Sydney Harbour.

Tom Slingsby's boat took out the first and last of four fleet races under twilight on Saturday, when variable easterly winds challenged the 11 F50 catamarans.

Australia sit second place on the event leaderboard with an eighth and sixth-placed finish to go with their two victories, so are on track for the event final contested by the top three boats.

But Sunday's final three fleet races will determine their hopes of competing for a first event win on Australian waters since the Sydney regatta in 2024.

"It would be incredible," Australia strategist Tash Bryant said.

"We won two years ago and that feeling is something that we really want back."

Still missing New Zealand and France after this month's horror crash in Auckland, the fleet rarely reached speeds above 60km/h in uncharacteristically light Sydney conditions.

"It's a tough race course, as soon as you're sailing in breeze that's coming directly off the land, you can't really look up the course and predict what's going to happen next," Bryant said.

"It makes it so hard. You really have to be in the moment, sail with what you can see and just hope it's going to go your way."

Australia continued their hot form from Auckland and pounced when Denmark fell off their foils in the first fleet race, crossing the finish line 75 seconds before the second-placed Danes.

The Aussies recovered from a boundary penalty and overtook the sputtering Brits on the fourth leg of the final fleet race, breaking away for a second race win.

But Australia were left to rue errors in the races in between, beginning the second race off the foils and finishing eighth.

Australia then conceded a costly penalty for starting the third fleet race too early, along with Germany and Denmark, so needed to drop back before a sixth-placed finish.

"It was a tough day, really hard day to be consistent," Bryant said.

On both occasions, Australia watched on as season four champions Spain made the better of the patchy conditions to win the second and third fleet races.

"We managed to get some good starts, which started our day quite well," said Spain skipper Diego Botin.

"(But) there's still a long, long way to go, no? Three more races before the final so a lot of points on the board to gain and lose."

Australia's star signing Iain Jensen returned from the knee injury suffered ahead of last month's Perth SailGP to make his debut for the Flying Roos.

The Olympic gold medallist was heard saying "knee's still good" on his headset microphone as Australia crossed the finishing line in the first race.

SYDNEY SAILGP LEADERBOARD:

1. Spain - 32 points

2. Australia - 28

3. Sweden - 26

4. Great Britain - 23

5. Italy - 22

6. USA - 22

7. Switzerland - 16

8. Germany - 16

9. Brazil - 15

10. Denmark - 14

11. Canada - 6

12. France* - 0

13. New Zealand* - 0

* Unable to race due to damage sustained at Auckland SailGP