It's contentious. Yes and no. In my opinion modern does compliment old, in most cases if done right.
The curvy nature does break up the straight lines in both the old Railway Station building (especially complementing the column design in various parts of this building) and the Festival Centre. Which I think is a positive.
The negative would've been if it was more like a block with vertical lines or horizontals (the Intercontinental and Riverside buildings for example) and the Walker proposal. The gold colour is similar to its surroundings (including the Convention Centre) except the Festival Centre but like an in between, so it's not too bad. I think it is good that the Festival Centre actually sticks out as white. It's separate. Good also to project images on the roof as done in the past.
The most impact this new building will have is towards the plaza and the Riverbank and across the river. It needs to standout at the end of the plaza. Hiding the heritage building, which loses nothing as it is a imposing grand building in its own right, is not too bad.
My only gripe is that the casino/hotel is too close to the old Railway Station building, which like Parliament House had a view to the plaza. It should've had more breathing space but hard to do in such a small area. My