i wanna say, the Bicentennial Observatory in the botanic gardens?
I like your thinking, that's not it though ... :)
Is that a bridge in the background? It would be unusual to light up the pillars and not the road, but I guess it's possible. (The shape is somewhat like Hindmarsh Island Bridge, but every photo I can find has the road lit and not the pillars).
I want to say the velodrome
The velodrome it is, taken from the City to Mawson Lakes path. The arch/dome structure was the key.
As the person who got it, have you a photo to put up Rusty?