Hi LenF,
Unfortunately … I don’t have any particular knowledge about the Australian market so I won’t be able to help much in terms of specific retailers or manufacturers I’m aware of there but the steps involved in choosing a mattress and the mattress materials that are used in Australia would generally be the same as in North America. I would also read post #2 here (except replace “Israel” with “Australia”).
Post #2 here also includes links to most of the forum topics from Australia and post #7 here includes a list of many Australian manufacturers and retailers and a few comments about some of them as well that will hopefully be helpful as well.
This is generally an indication that a mattress isn’t keeping your spine and joints in neutral alignment over the course of the night so instead of the mattress doing the work of keeping you in alignment your muscles are doing the work.
The best way to avoid this is with careful testing (hopefully using the testing guidelines in the tutorial).
Phoenix