The prescribed ratio in the Worker Engagement,... regs is 1 H&S Rep for every 19 workers (R6)

My understanding is that elected health & safety reps and staff health & safety committee members are two entirely different positions with different responsibilities. Unless there is a special agreement with management, staff and unions that elected H&S reps will make up your health & safety committee. This would be highly unusual.

Unless there is a special agreement with management, staff and unions that elected H&S reps will make up your health & safety committee. This would be highly unusual. — Tony Walton

The regs require that at least half of the H&S committee members must be workers representing the workers (and they have not been nominated by the organisation). It also clarifies that H&S Representatives are eligible to be a member of the committee.
Given the H&S Representatives are generally workers who represent the workers (or workgroup) it seems common practice that they are members of the committee. The only stumbling points would be if the organisation is selecting (either overtly or covertly) the H&S representatives and not the workers, or if the H&S Representative is not a worker of the organisation itself (but the last one would only mean the H&S rep would count toward the required half of members).

this ratio is prescribed for the workplace that only has one workgroup. If the workplace has more than one work group there are a list of things in r7 to consider. In practice 1:19 is a handy guide

I also find it useful to make this distinction. Worker engagement can and should go beyond reps. Not all reps need be on committee and not all committee members need be reps. A committee member resigning from committee business leads to a the filling of a committee position according to agreed process; a constitution can be helpful. A representative resigns from representing the work group is unrepresented until that is resolved to the already agreed process as per the H&S engagement reps generally and r13 in particular.

Notwithstanding any ratio by regulation, I believe the most important factor is that every worker/ staff member confidentially feels that they are personally represented on the H&S Committee and by a Representative that they relate to and who knows their particular job and their part in the structure of the organisation.