Generally speaking, you should keep savings in the currency that you intend to spend the savings in. Otherwise, you are taking on big FX risk.
Deposit rates for AUD are much higher than for EUR because the central bank base rate is higher in Australia.
I suggest:
(1) Convert the AUD to EUR as soon as possible, make sure you get a good rate, consider using an online currency broker such as Currency Fair.
(2) Deposit the EUR into a Nationwide UK (Ireland) 30 day notice @ 1.75%.