The Building code for slip resistance for pedestrian access routes states that surfaces which provide the direct access route (including where this is a deck) require a slip resistance of 0.4.
The Department of Building and Housing website has an overview regarding slip resistance
The Building Code compliance document D1 outlines the requirements for access routes, and states in Table 2 that timber is OK in dry level conditions, but needs an across-grain profile or sand/grit finish for wet and sloping zones. Algal growth over time would need waterblasting as a maintenance scheme, as per subclause 4 of table 2. Alternatively, unprofiled timber can be used with a fixed weatherproof coarse matting providing slip resistance of 0.4 for the main access route across the deck.