The copyright act of Australia set out the rules about copyright ownership.
This is really quite straight forward. Unless there is an agreement between the client and photographer/s, then the client owns the copyright, if the photo’s are commissioned for “private or domestic” purposes. This includes weddings, portrait shoots etc.
If a guest takes photo’s at a wedding then they will own the copyright.
If there is an agreement in place between the client and the photographer then you need to make sure you read and understand the terms and conditions of the contract before signing it.