The offer is on homepage of the uber eats app.
If you see a map when loading uber eats website, click delivery on the top to see the banner.
Also, scroll down a bit further and there is a "Today’s offers" where there are more deals.
There will be different restaurants throughout the day as they open. I had a bubble tea place yesterday afternoon but I cannot see on the list yet as they open at 12pm.
To redeem this offer, search for a participating restaurant with a ‘Buy 1, Get 1’ tag, add the promoted item to your cart and place your order. Delivery Fee and Service Fee applies. Offer available Monday through to Thursdays between 13 and 30 September 2021 in Australia and New Zealand where the Uber Eats app is available. Subject to availability in your area and operating hours and stock availability.