It's a bit dressier than I'd normally wear for a daytime outdoor wedding, but since the reception is evening, it looks fine. I like the suggestion upthread of dressing it down slightly for the ceremony and back up for the reception.
I pretty much agree with this. I do think it would be too dressy if everything were in the afternoon. But if the evening reception is to be dressy, then I think it's fine for the ceremony, too.
But I would ask if they think people will be wearing long gowns or short party dresses or something less dressy than that. Especially because you said that on you the dress is floor length, not ankle length. I would feel self-conscious being the only guest in an evening gown when the other guests aren't even in evening clothes, just nice daytime dresses and pants. I don't know about Australia, but in my community, unless the invitation says "black tie," no one would wear a long evening gown.
If only a few women will be wearing party clothes, maybe you could shorten it a little so it's more like a maxi, and wear more casual, daytime shoes and cardigan with it for the ceremony, and then switch to party shoes and jewelry for the reception.
(Probably my advice would be not to go with a long dress under these circumstances, but I really love that beautiful dress!)