You might have heard that it is illegal to buy electronic nicotine cigarettes in Australia.
But this is not true. You CAN, in fact, buy e-cigarettes containing nicotine if you live in Australia. Ordering a concentrate from overseas and putting a "shot" in your e-liquid is the most commonly practiced way.
Once you have your concentrate a quick google search for a calculator will tell you how much to mix for a desired strength.
If you order the 100mg variant, you can use this handy chart.