Upvote:2
If you can’t remember the exact date and/or don’t have any paperwork relating to the rejection, you should enter your best ‘guesstimate’ and provide an explanation in your covering letter.
Having said that, if time permits, I would email your local Australian Embassy https://immi.homeaffairs.gov.au/help-support/contact-us/offices-and-locations to ask how to get details of your Australian immigration record. A quick search of the Australian Dept of Home Affairs website didn’t reveal any other way of obtaining the information you need.