Untreated timber left to the elements is likely to grow mould.
It doesn't look too good on a deck but we use blackbutt for timber boardwalks on beaches because of its durability. The heat killed the mould. If we had a lot of wet weather the mould would grow. The untreated timber turns a silvery grey out in the elements. We expected it to last about 40 years, untreated and out in the elements.
The Hunter is a warm place and there is a reasonable humidity. Good conditions for mould to grow.
Any finish will keep the moisture out. No moisture - no mould.
Staining won't make any difference.
Clean up the deck by sanding or a commercial mould remover. You need to get rid of as much of the mould as possible so I would suggest a mould remover.
Let it dry if you use a mould remover.
Apply your finish and the deck will look a million dollars for years.
Give another coat when it looks like it needs it.
The pics are of blackbutt flooring, also in the Hunter Valley.
It comes up pretty good.
It has an Intergrain finish. They have an outdoor version. Feast and Watson have a good outdoor finish that I like because it contains tung oil. The finish can be almost flat to gloss. There are good oil finishes available too.
Clean it up, put on your finish and will enjoy it for years.
The ark was built by an amateur
the titanic was built by professionals