Rightly or wrongly, creditors and debt collectors who do this are not stepping outside the law.
If you are out of communication with them, they are within their rights to use this avenue (and a variety of others) to attempt to contact you.
Yes, they will likely contact family and friends in an attempt to contact you! However, they are legally not permitted to reveal details of your debt to a third party.
For more information on how debt collectors work, visit the Australian government/ASIC website Moneysmart at:
https://www.moneysmart.gov.au/managing-your-money/managing-debts/dealing-with-debt-collectors