There's a quasi-height limit on Southbank of 300m. Eureka's ground level is 2.7m above sea level (the Yarra etc) and the tower height is 297.3m.
I say quasi as height "limits" can be exceeded if the design is "exemplary" [which is subjective], but afaik the planning overlays in the Capital City Zone (the CBD + Southbank basically) allow this.
No-one's pushed it as yet, but the last remaining site where this could happen is, funnily enough, directly infront of Eureka
^ in the above pic, the three low-rise campuses (Which were some of the original offices onm Southbank from 15-20 or so years ago) + a multi-level car park just behind them (where most of us took construction update pics of Eureka way back when!) could theoretically consolidated and an enormous tower the size of Eureka (or taller) built on top
the funny thing about this one is it's the same developer as Prima-Pearl which is 254m tall. On OzScrapers a few pissed off buyers of Prima apartments are already bemoaning being duped by sales staff
that the site couldn't have something like this proposal built on it - which is total bullshit as it's all the same zone and everything's fair game as the height limit so close to the river is mute - which is the southern bank of the river cannot be overshadowed in the middle of July (remembering the sun is in the northern sky down here, and the tower is on the southern bank of the river already
). furthermore, a tower like Rialto couldn't be built in the lower CBD in this day and age owing to that restriction (Which came into force after Rialto and Optus on Collins Street (which is the other tower which overshadows in the middle of July at the moment).