Australia post indicate that regular letters which would cover your ballot papers can take the following times to reach their destination.
Same state: 2-4 business days
Interstate: 3-6 business days
If you intend to cast a ballot and return it to the Australian Electoral Commission in the Pre-Paid envelope you will need to ensure that they receive it by 10am AEST on the 1st May 2024
If you need to call the AEC, they can be contacted on 03 9285 7111