Our recommendation would be a dense hardwood that will absorb moisture slowly, but the level of moisture absorption will depend on the length of time it is immersed. Tallowwood would be a good choice since it has a naturally greasy content that gives it its name. Ironbark would also be a suitable choice. If flooding is expected fasteners will need to be carefully chosen to avoid corrosion, and would preferably be stainless steel. Decking oil will be easiest to maintain, but will need regular application since it will not soak into the wood to any depth.

We do not have specialised knowledge of metals but we are not aware of any adverse reaction when steel that is coated with zinc-rich paint comes into contact with wood. If the wood is treated with a copper-based preservative and exposed to moisture there is potential for traces of copper to leach from the wood and attack metal fasteners. This is due to galvanic corrosion where dissimilar metals are in contact in a moist environment. Conversely if metal fasteners corrode, the corrosion products degrade adjacent wood ('iron sickness'), but this also requires the presence of moisture.

We weren't sure which installation guide was followed, but your image doesn't show obvious fixings. Boral have a comprehensive cladding guide which includes the advice "Fixing is to use two fixings per crossing". ie. two nails or screws through the face at each point where the cladding crosses a batten. We consider this to be standard industry practice.

Some years ago there was an Australian Standard titled Types of timber surfaces, published as AS 1728-1975, but it was withdrawn in 2017. The only current definition we can refer you to is in Australian/New Zealand Standard 4491:1997 titled Timber - Glossary of terms in timber-related Standards. "Dressed Timber" is defined as "Timber finished to a smooth surface on one or more surfaces". The term DAR means dressed all round, ie. smooth on all surfaces. In our opinion the image attached to your question does not present as timber "smooth on all surfaces".

Our Timber Service Life Design Guide will give you the probable lifespan of a merbau deck in Melbourne. It's number 5 in our series of Technical Design Guides, and can be downloaded free of charge from our website via this link https://www.woodsolutions.com.au/publications. Merbau is rated Durability Class 1 outdoors above ground, and Melbourne is in Climate Zone C according to the Decay Hazard map in the Guide.