Hi Caz,
Your best bet would be to speak to the bank you are with here in the UK first and see if they have a relationship with any of the main banks in Australia.
We are looking at switching from Barclays business to Lloyds as they are partnered with ANZ bank in Aus who we want to bank with.
There shouldn't be too much hassle in opening a business account and merchant services with an Ausralian bank providing you have all the necessary requirements/documents etc.
The main question to ask is are you registered to operate a business within Australia as this will be the main barrier to getting merchant (credit card processing) services in Australia.
Also, WorldPay are excellent (as you probably already know) and also operate in Australia, see
http://www.worldpay.com/aus/There are a lot of differences between the merchant services in banks so be sure to check out a few of them before you decide. Some of the main banks are: Commonwealth Bank, ANZ, Westpac, National Australia Bank, Bendigo Bank just to name a few.
Good luck!