This billboard is still on display up high above Flinders Street, opposite the station, roughly across from the centre entrance*.
It seems to refer to the 20% emissions reduction by 2020 pledged last year by Labor, and matched by the Coalition, though some say there are indications the Coalition will drop the target.
Perhaps it’s just too difficult for the advertising company to remove it, and/or they haven’t got another client who wants the space. Despite the size of the sign, it’s not really very noticeable.
Google Streetview shows a different advert up there.
(Tip-off: Mike Alexander.)
*At least, it was still there a few days ago. I haven’t checked this week.