I don't agree with Hoppy or your builder. In Adelaide I'm not sure why you'd bother as there is low rainfall and low humidity. If you really did want to bother then simply use a good external 'no prime' acrylic paint and whack on two coats and only on the flat underside not the edges- no need to waste an expensive decking coating on what will never been seen. But the life of your decking will be down to how you initially finish then maintain the top surface facing the sun and the weather. Do it right and a deck can last 30, 40, 50 years, do it wrong and it'll be buggered under 10. But since most people move houses every 10 years or other decks tend to get used by the owner who built it and the next owner - the 3rd owner does a reno and rips out the deck . . . even if it is in OK condition.
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.