I don't understand exactly what you need: "only need phone for whatsapp and accomm" and "want to keep my normal number".
In my experience, Whatsapp is the most commonly used method to book accommodation.
If you need to be able to receive calls on your Australian number, as was suggested before, perhaps you can leave a recorded message to contact you via email or whatsapp instead while you're away. This will save you a lot of money, as you can avoid roaming altogether.
In case you didn't know, if you swap the SIM card for a local one, all the whatsapp contacts, chat history, etc. remain the same. People can still reach you through whatsapp using your Aussie number, even after you swapped the SIM.
You must ensure that your phone is unlocked though, for the SIM swap to work for you.
If you go the SIM swap route, don't buy your Spanish SIM at the airport (it's a rip-of). I flew to Madrid, took a bus to Atocha station (5 Euro), and bought a Vodafone SIM at a shop there. I think it was 20 or 25 Euro for 28 days including plenty of data, and it was easy to recharge online when it was about to expire.
For your banking needs, I know that some Aussie banks will only use SMS to send you a verification code, but with CBA and Suncorp can be set to work with an app instead. All your apps still work after you swap the SIM.