I suffer with chronic back issues as well but never have concerns with the beds in any ACCOR property in OZ. If you're worried - advise reservations to inform the housekeeping dept.
Beds are ok, had better . I would recommend the Sofitel darling harbour, New so new quality beds and better hotel.
If I remember correctly they were fine. No issues at all.
Beds were comfortable but as we don’t have back problems we cannot comment on how your back will manage. MY be best to ask the hotel directly.
Very comfortable and firm good for a bad back,You can always contact the hotel and ask for a firm bed, I am sure they will be able to accommodate