AS1684 is specific on ceiling batten sizes but, as you say, doesn't specify how to fasten them. If the ceiling battens are part of a trussed roof system, providing bottom chord restraint, the truss company should be able to provide connection details. Installation guides for products such as Gyprock assume the plasterboard will be direct-fixed to the underside of ceiling joist or trusses, in which case the plasterboard has the same restraining effect as ceiling battens at 600mm centres.

There is no standard time limit on the allowable weather exposure of timber framing. Perhaps moisture content has exceeded 20% for periods of time, with periods of drying in between, since it would be unusual to record such high levels of moisture for a continuous 12 month period. It might also be the case that the high moisture levels are only present in surface layers, rather than throughout the depth of the timber.

New Guinea Rosewood is relatively durable outdoors, rated Class 2 above ground according to Australian Standard 5604, Timber – Natural durability ratings. This implies a probable service life of 15 to 40 years. The upper limit of 40 years depends on the timber being subject only to ‘periodic moderate wetting’, with adequate ventilation and drainage, while a lower service life would apply if wet for prolonged periods, eg. weather-exposed in a tropical or sub-tropical climate.

The regulations dealing with external wall cladding are set out in the ABCB Housing Provisions which can be accessed via the relevant website. Section 7.5 deals with timber cladding and focuses largely on adequate fixings and sarking, rather than species. Manna gum and messmate are rated Class 3 durability outdoors above ground according to Australian Standard 5604, Timber - Natural durability ratings, ie. they are not highly durable species. However, neither are some other species traditionally used for cladding, such as Baltic pine.