NCC 2022 (effective 1 May 2023) deals with this in clause D3D4, vol. 1 page 197. A non-fire-isolated stairway, not required to be in a fire-resisting shaft, may be of timber if it is of finished thickness not less than 44mm, density not less than 800 kg/m³, and not joined with glue unless laminated with a formaldehyde-type glue.

AS 1720 provides load capacities for screws loaded in shear and withdrawal in both seasoned and unseasoned timber. The loading data in AS 1720 applies to Type 17 self-drilling screws, including bugle-head screws. Manufacturers may have their own data, eg Buildex, see here - https://www.buildex.com.au/images/documents/03125-PDS-new.pdf. It is advisable to check that the capacity of the screws you are using matches the capacities required by AS 1720 for the particular timber Joint Group.

Australian/New Zealand Standard 1604.1:2021, Preservative-treated wood-based products, approves H2S treatment for protection against borers and termites "inside above ground, protected from wetting". If you are confident that there is no possibility of moisture ingress so the deck is comparable to being in an indoor environment, then perhaps H2S would be suitable. However, we would prefer to see a minimum H3 treatment to provide protection against decay, combined with ventilation, in case of condensation inside the fibre cement enclosure.

It's going to be a two-stage process, first to identify the species of timber (if not documented anywhere), and then to determine a stress grade. To identify timber species we usually refer to Dr. Jugo Ilic in Melbourne. It’s not a service WoodSolutions can provide.