I searched OZB and couldn't find any cheaper tickets for a single adult. Of course there are cheaper tickets for the family or 2 people etc., So, if a single person wants to go to the Melbourne Aquarium, this might be a good deal. Cheaper than the 20% discount that's possible for most of the days on the Aquarium's website itself.
The tickets can be purchased from Peterpans Travel, Shop 4 Northbank Place, 565 Flinders Street which is about one minute walk from the Aquarium. According to the staff there, these tickets are open dated and can be used on any day until the end of March 2018. They also said that the tickets can be purchased on the day and use them straight away by taking the voucher they give you. You can also go through the prepaid line with these vouchers. According to google, their store is open Monday to Friday from 10 am to 6 pm and 10 am to 5 pm on weekends.
Hopefully this will be useful for some solo adults who want to visit the aquarium.
Cheers
https://www.youtube.com/watch?v=yz2DYaMJ4ss