I have been bitten by something it was immediately itchy an hour later tender, swollen, still itchy and sore, with a donut shaped purple bruise ?

Wash,and Clean: The area well, Apply Antihistaminic and Antibiotic cream . Take Anti inflammatory medicine. Alsoapply Ice to the area. If continues to spread, please get examined by MD for more aggressive treatment.

I know Australia has: some mean critters down under.Benadryl (diphenhydramine) or any antihistamine may help with itching.Observe closely for development of infection,which would necessitate the addition of antibiotics.U may show this peculiar bite mark to someone familiar with local flora/fauna for further risk assessment.Some bites are self limited and others can have longer term consequences like parasitic infestation,for example.