The building code specifies standards for slip resistance of pedestrian access routes. 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 the basic information in a comprehensive format.
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 will require 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.