It's important for your clients to understand that the shrinkage figures quoted on our website for spotted gum (tangential 6.1%, and radial 4.3%) refer to the shrinkage that the timber undergoes in drying from the unseasoned condition down to a moisture content of 12%. No doubt the decking boards used for the screen will be kiln-dried, ie. 'seasoned', so their moisture content at the time of installation will already be around 12%, in which case shrinkage will be minimal unless there is a long period of hot dry weather.

AS 3959-2009 is now superseded by AS 3959-2018, but the focus is still on preventing fire from entering the building envelope, rather than on preventing its spread internally. So there are no restrictions on internal wall construction, nor on the internal face of an external wall. Regarding FRL's associated with external walls, in BAL-FZ one of the options for a timber-framed external wall is to utilise a system with an FRL of -/30/30 when tested from the outside. Such a system would also satisfy external wall requirements in a lower BAL (clause 3.4).

Our Technical Design Guide no. 21 titled Domestic Timber Deck Design will be a useful reference,  available for download free of charge via this link https://www.woodsolutions.com.au/publications. The three timber species you mention are all 'durable' in both senses of the term - ie. wear resistance and rot resistance. All three fall into Durability Class 1 in terms of AS 5604, Timber - Natural durability ratings when placed in above-ground situations.

Oriented Strand Lumber (OSL), more commonly termed Oriented Strand Board (OSB), is not marketed as a structural product and hence doesn't have a stress grade according to the Australian system. Strength properties are contained in a US document available via this link https://www.pfsteco.com/techtips/pdf/tt_osbdesigncapacities.

AS 1530.3 is titled Simultaneous determination of ignitability, flame propagation, heat release and smoke release. These values are collectively known as the ‘Early Fire Hazard’ properties. For blackbutt the numbers are: Ignitability 13; Flame Propagation 6; Heat Release 5; and Smoke Release 3.