From my personal experience, my Polo with 15" factory alloys has far far better handling than my friend's upscaled aftermarket 16" wheels.
Frankly on Indian roads, the handling becomes laughable beyond 16".
Factoring in Australian roads, which must be smoother, I would still not recommend going 18" as you will feel even the smallest pebble on the road and won't give you any handling advantage apart from better looks. You may have to upgrade your springs possibly?
Stick to 17" and get suitable tyres.
I would like to recommend my favorites:
Team Imola 17" White
Team Imola 17" Black
They also have 18" versions on their website if you insist.
How they look?
Here
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