We assume the timber panelling "outside the unenclosed shower area" won't get wet when the shower is in use. Regular wetting is not a situation we would recommend. However, if it's just a case of a species that will tolerate humidity and changes in temperature, western red cedar would be a good choice. That's why it is the preferred choice for saunas. Also we trust there would be an exhaust fan in the bathroom, preferably one that operates automatically. A suggestion regarding the base of the wall is to consider a row of tiles as a skirting, with the timber above. 

This is a question that should be put to the Standards Australia Technical Committee responsible for AS 1684.2:2021. The same wording appeared in AS 1684.2:2010 but we don't have data on the basis for the clause. Lintel trimmers, of course, are different from windowsill trimmers in that they act in conjunction with the lintel. Design deflections of lintels in combination with lintel trimmers have been determined to maintain clearances between the house frame and the window or door frame. Windowsill trimmers act alone.

It's very difficult to estimate the life of a western red cedar shingle-clad wall in the Melbourne climate, since it depends on weather exposure (whether there is any shielding), roof overhangs if any, maintenance of a finish and similar factors. However, some general observations might help. Timber-clad walls survive best when water runs off without being trapped. Western red cedar is a durable species and is rated Class 2 outdoors, above ground, according to Australian Standard 5604:2022, Timber - Natural durability ratings.