This brand seems to be getting very good reviews (the low price could be part of the reason for that, but the reviews I read mentioned quality rather than price, so it's probably not a big reason).
King size Euro Top Pocket Spring $99 (was $299)
(Sold Out) (Available Again)
Queen size Bed Memory Foam Hybrid Mattress $179 (was $379)
Please note that there are a number of reports in the comments that delivery is not available to some areas.
But, before you click, be aware that delivery cost can be very substantial, and varies by factors such as weight of product.
So, to see how much it would cost to be delivered to you, add the mattress to your cart (there is no need to log in or create an account to do this) Click checkout, and enter your postcode.
The delivery fee I looked at, to a suburb about half an hour out of Brisbane which is about 30-35 Km, is almost $80 (which made the total cost of the $99 mattress closer to $170).
Subject to availability, this product is available for purchase in NSW, QLD, VIC, WA, SA and TAS, and the North Island of New Zealand.
Note: this is basically the same deal as one they had a few months ago https://www.ozbargain.com.au/node/648220 (except the full retail price was $399 instead of the $299 it is now).
But it's only very few sizes that get such big discounts, which makes me think that the mattresses are not bad, but the sizes don't sell?