Digital Rights Watch has more information and a submission system to help people write their feedback to the government: https://digitalrightswatch.org.au/2018/08/19/defend-encrypti...
The parent's link, allows you to post a boilerplate submission with a single click. Far better to write and email your own submission, as form letters tend to get aggregated into one during evaluation. Your own submission only has to be a few lines, even if it just paraphrases a form submission. Uniqueness counts over bulk submissions.
The page for the inquiry is:
https://www.homeaffairs.gov.au/about/consultations/assistanc...
The email address for submissions is:
AssistanceBill.Consultation@homeaffairs.gov.au
Less time critically, you also need to write to or call your federal MP, but I'd suggest that a personal submission to the inquiry is the most "bang for buck".
The real question’s not which one of these paths to chase;
but whether you can pursue them all, in a balanced way,
so that they keep each other in check with restraint
without letting any one of them dominate.