Volitan

Pennant Fish ( Alectis ciliaris)
 
Diamond Trevally (Alectis indica)
 
Closely related and hard to tell apart. The best distinguishing feature for juveniles is the ventral fins which have elongated filaments in the diamond trevally, but not the pennant fish.
Also, juvenile pennant fish photos do indeed show a black spot at base of dorsal fin, but that is not mentioned in my books. I’m not sure why but I’m guessing it’s because it’s not always present - or perhaps because scientists generally don’t like to use colour as a distinguishing feature.
So its a pennant fish. Also called African pompano and many other names which you’d expect for a fish that occurs worldwide.
On whether it’s rare or not, the official range of both species is to Wollongong. Apparently juveniles ride the East Australia current down the coast in late summer and are resident in estuaries in the Sydney area, but die off as the water cools. So not rare, but not common either.

Looking at the book Sea Fishes of Southern Australia.
The main diagnostic is the rear margin of the first dorsal fin. Concave in sand whiting, convex in trumpeter and eastern school whiting.
Also mentioned is the ‘dusky blotch at the base of the pectoral fin’. Present in sand and trumpeter whiting, absent in school whiting.
Also mentioned are the diagonal bars along the back of the school whiting - which darken after death.
So its diagnostics are in line with a sand whiting.

No, I hadn’t, but following your suggestion I worked the key through and came up immediately with this
 
black cube head, a type of driftfish
Cubiceps baxteri
or a related species - it seems there are several species in Australia.
 
This is the page I ended up at
http://fishesofaustralia.net.au/Home/family/32
It would take someone better at ID then me to be definite, though it seems a very close match on all the features I noted or photographed, and the general overall look.
thanks for the tip-off - I’d seen that key before but had forgotten about it.
 
cheers
V.
 

Yeah, weed fish for sure. We’ve had one here on the forum recently. There’s a handful of local species and they’re hard to tell apart being so variable but I’m going to call it a rosy weedfish (Heteroclinus roseus) due to the little black and white mark on the forward edge of the pectoral fin base. 
 

Have a google on ‘ratfish’ or ‘chimera’ or more accurately ‘chimaera’.

my first thought as well, but I rejected it on the basis that:
Pectoral fin on longfin perch is much longer - extends to midway of rear dorsal fin. 
Yellow fins are characteristic of Eastern orange perch both sexes, not female longfin.
The Eastern orange has one or more darker lines paralleling and above the lateral line.
The Eastern orange has a white trailing edge to the outer gill cover.
 
Eastern orange
http://fishesofaustralia.net.au/home/species/4380
female longfin
http://fishesofaustralia.net.au/home/species/4380

Starry toadfish. Arothron firmamentum.

Right hand fish in last photo is a sand mullet. The orange spot in front of the pectoral is diagnostic, as is the round black spot at the top of the pectoral fin base. In the sea mullet, the pectoral spot is blue and more elongate.
the one on the left is harder to see - can’t be sure.