Reverse cycle air conditioning does have a drying effect on timber - in fact drying timber in dehumidification kilns relies on this principle. Your additional details via email indicate that the house in question is at Wodonga in northern Victoria and will be well insulated, with air conditioning only operated intermittently. CSIRO advises that in Melbourne in an air conditioned environment timber moisture content will range from 7-10% with an average of 9%, although in an uncontrolled indoor environment it will range from 10-13%.

We understand you have already accessed the CSR Red Book for details of System 2340. If the Red Book doesn't provide the information you need, or you require a fire test report, CSR's DesignLINK Technical Support Team may be able to help, phone 1800 621 117.

Table D2.14 of NCC 2019 calls for several different slip-resistance levels, depending on the slope of the ramp and whether dry surface conditions or wet surface conditions apply. Assuming a slope between 1:20 and 1:14, and assuming the ramp will be exposed to the weather and therefore in use when wet, P4 slip resistance would be required. You will find various slip-resistant decking finishes on the net that achieve a P4 rating, including Intergrain UltraDeck, Sikkens Cetol, etc.