How can I report animal cruelty?
First of all, thank you for looking out for the welfare of animals.
If you have witnessed an act of animal cruelty, your best avenue of contact will be the RSPCA as they have the authority to investigate and prosecute on matters of animal cruelty and neglect. Contact details for your state can be found here.
Your local council may also be able to offer some advice to you, so it would be worth contacting them as well.