Well, turning me head upside down it appears you still have a fairly strong leader there, so I'd suggest that if you have a fairly long stake I'd bang it into the soil and pull that strong branch, gradually if it looks like it might break at the junction, straight. If you haven't, try a sturdy stick tied to the main stem below the branching and again to the branch above the stem.
B

PS. Also, if you have other branches putting out strong growth, I'd restrict them by cutting them back by a third which should encourage growth on the branch you want as a leader.

I hate to say it, but I would not have chosen that tree for your yard. It is potentially very big - big enough to cause root damage both in your yard and to the neighbour's driveway.
You will also be needing to attend to overhang into the neighbour's as well as dealing with the mess of fallen leaves as they are semi-deciduous in Qld.
I also don't know that they respond so well to pruning that you can "shape" them.
That said, the GC city council gardens have a few Cassia brewsteri (or some cultivar) growing along the entrance driveway and they have managed by constant trimming of the lower branches to get them to form a nice "avenue".
And I do LIKE the tree - in fact it's one of the most spectacular flowering trees... in the right spot.
I planted one in my front yard years ago but had dreadful problems with branches being ring-barked by insects and then infested with some tiny white gnat-like things that were stinging the new leaves and causing them to shrivel and fall off.
Eventually I gave mine the chop to ground level but as I didn't poison the stump it shot some new growth and I let it grow back. I can say from that experience that lack of a single leader wasn't a problem for growth and eventual flowering, though I suspect some of the uprights are a bit sappier and weaker.
that wouldn't be a problem with yours, but as I say, I would re-think your choice while it's still young enough to remove.