Your info says your from Sydney so your obviously bound by the NSW keeping system. According to the Class 1 licensing system there are 11 turtles/tortoises you could keep.
Here are all 11 species along with several adult lengths.
Eastern Snake-necked Turtle (Chelodina longicollis) 20-25 cm
New Guinea Snake-necked Turtle (Chelodina novaeguinae)
Oblong Snake-necked Turtle (Chelodina oblonga)
Northern Snake-necked Turtle (Chelodina rugosa) 30-40 cm
Northern Snapping Turtle (Elseya dentata) 25-35 cm
Saw-shelled Turtle (Elseya latisternum)
Mary River Turtle (Elusor macrurus)
Cooper Creek Short-necked Turtle (Emydura sp.)
Krefft`s Short-necked Turtle (Emydura kreffti) 20-25 cm
Murray Short-necked Turtle (Emydura macquarii) 20-30 cm
Brisbane River Short-necked Turtle (Emydura macquarii signata)
("A Photographic Guide to SNAKES & OTHER REPTILES OF AUSTRALIA", Gerry Swan. 1995, New Holland Publishers, Australia)
I hope someone can help out with the other 6 turtle`s lengths. Ive never kept turtles before (planning on in the next year sometime) but I`m pretty sure they don`t reach adult size until maybe 8 or 9 years. Hopefully someone else whos had experience keeping turtles can verify/disprove my statement.
Cheers, Jay