"Are there any current or former RAN pilots out there? Is it unusual to approach a Ships helo deck downwind?"
Normally the flying course used in launch/recovery is one that provides 20-40 knots wind over the deck within an envelope. The envelope varies around the ship, based on aircraft type, night day ect. The most common would be within 40 degrees either side of the direction a ship is heading with relative wind of 20-40 knots.
As you are using a relative wind to find a flying course you can end up with an approach that has a downwind component and still be within the envelope (shol) Not preferred by too many aircraft at night though