These boxed mattress companies all have promos at the moment.
Sleeping Duck (https://www.sleepingduck.com/au) - $100 off, code: ENTE
Greywing (https://www.greywing.com.au) - $200 off, code: GREATTOMORROW
Ecosa (http://www.ecosa.com.au) - $250 off, code: WINTER
Onebed (https://www.onebed.com.au) - $300 off + free onesie, code: ONESIE300
As an example, this brings the price of a queen mattress from each company down to $1249, $995, $849, and $699 respectively.
Check here for some info regarding boxed mattresses: https://www.ozbargain.com.au/wiki/mattresses