Under strata law, if you are repairing the carpet, then you need to replace like with like. So as your old carpet lacked safety treads, then so should your new carpet. If you are upgrading and improving common property, which adding stair treads to the regular carpeted stairs, would most likely be considered to be, then that requires approval at a general meeting as a special resolution. If the repair was approved at a general meeting, as an ordinary resolution, and was described as a “repair” then you would not have justification for spending additional money on improving the stairs. Repairs are paid from the Administration fund, while improvements usually are paid from the Capital Works fund, too.
Nevertheless, if your building should arrange for a Work Health and Safety Report, and in that report the stairs are faulted because they lack treads, then you would be obliged to quickly install the treads, and improve the building’s common property. I doubt that a WH&S inspection would fault stairs without treads, as this is the commonest situation, unless there are some special circumstances involved. If some owners are keen to add treads, then they could arrange for a WH&S inspection and report (which strata buildings should be doing regularly), and make a case to the inspector that the stair are unsafe in their current condition. The inspection may or may not be convinced…