indeed it is a pelargonium (technically) and they originate from my part of the world. So they are adapted to plenty of rain in winter and long dry summers. Full sun is best or very light shade. In fact the unhybridized ones are as close to a succulent you can get without actually being a succulent. Yours look pretty unhybridized, does it perhaps have swollen storage roots, almost tubers? If it does than you know it is accustomed to long periods without water.
In the wild they are often browsed by small stock and so in the garden most of time with them I spend imitating a goat. In fact one of my favourite chop and drop shrubs is a pelargonium which gets no irrigation at all, I normally chop it in autumn just before the rains come on and it makes a most lovely soil. It looks to me like your plant got leggy because it has not been browsed regularly. If you prune it gently it will also bloom better. Don't try to do all at once now that it is unused to pruning, prune at most a third at a time so that doesn't go into shock. Leave it for a couple of weeks and then prune another few branches. All the cuttings root easily and it looks as if one of your branches is trying to root in the mulch. They do like a fresh soil every couple of years or lots of nitrogenous mulch (think goat droppings :0)
On another note I have been watering my fancy potted pelargoniums with garden kombucha (stinging nettle, alfalfa, kelp) They perk right up and bloom for weeks on end. So check if your microbiology is right. Cape plants are notorious for being picky about their mycorrhizal associations, so much so that a few will not grow outside the Cape or even outside a very limited region here.
A belated "thanks" for your replies, we really appreciate it.
I had also posted onto a Facebook Plant ID group, and got the same info I got here - Pelargonium sidoides
Looks like quite the amazing plant and it's been fun to learn more about it. We didn't actually plant it, it
just appeared one year. We get all sorts of odd and ends in our yard thanks to the birds, squirrels etc... and
this was one of 'em.
We've moved it out from under the shady oak tree and into a more sunny locale, so hopefully it takes.
Probably need to figure out how, or if, to prune it properly, or if we should just leave it be.
Location: Australia, New South Wales. Köppen: Cfa (Humid Subtropical), USDA: 10/11