$-Euro on 17/11/13 was $0.7417
yesterdays average was $0.8987
difference of just over 21%
$-Aud on 17/11/13 was $1.0673
yesterdays average was $1.3216
difference of just over 23.6% (closer to 22.5% using your figures)
Not such a big difference after all, besides which currency pairs are not tied to one another in such a way that prices exactly correlate.
Regarding your question, surely if you live in Australia you have to spend in Aussie dollars, unless you are buying products from overseas in which case either the local currency or, frequently, US dollars are acceptable