I apologise for the poor quality of the pictures; it was a bit too sunny, and I had to use the zoom (I didn't want to disturb the spider). I'm fairly certain that it's in the genus Argiope, just not quite sure of the species.
The genus is known locally as "signature spiders" (aranha-assinatura), but that's basically any spider doing that anti-bird web pattern. It's yellower than the pic implies, colours look a lot like A. bruennichi, but those side "lobes" are quite different.
Behaviour-wise it's surprisingly chill. It doesn't run away like brown recluses and wolf spiders do, nor threatens you like wandering spiders (all three are native here).
The web is clearly strong enough to slightly bend one of the kumquat's branches.
I also got a pic of the spider having a meal. Perhaps the "web bedroll" around the prey might be useful for ID.
The species is actually A. argentata acc. to another poster, but still likely a female, so I'll definitively post pics if she lays eggs. (She's now Kumoko.) They aren't dangerous - acc. to some websearch they only bite if hurt, there are plenty pics of people with them on hands, and even if they do bite the venom is comparable to a bee sting*.
and jealous that you have a kumquat tree
It's a dwarf nagami variety, so a bit more like a bush:
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*I wish that I could say the same of these:
Gaucho spiders. Native and fairly common in my homeland. Strong venom and a tendency to hide inside wardrobes, their only saving grace is to be more scared of humans than we are of them.