I've just commented on this in another thread, but I'll expand on the comments here as well.
There are so many apartments going up in the CBD and surrounding suburbs it isn't funny. I'll take your word that UWS stage 1 is sold out, because I don't know myself. Keep note that there are two other towers (I believe) to be built on this site (including the one being halfway up now). Add this to the four towers to go up on the old Age site opposite, plus I think there may be more going up on the corner of Spencer and La Trobe, and that could eventually make something like four thousand or more apartments in the space of one block. That is a hell of a saturation for an area which is arguably the quietest part of the CBD.
Don't always assume that because development is occurring that that it is because the demand is there. When these big developers see in the news and in demographic statistics that there is a shift of people moving to the inner city, they get greedy and build as much as they can, expecting to meet the current/short-term future demand. But in most cases, they always build way too much. Sure, once the developments die down, in five to ten years you may eventually begin to see the level of supply vs demand come down, but in the meantime you will suffer low or negative growth, and high vacancies. Happens everywhere.
This is why these developers offer these absurd rental guarantees. They wouldn't need to offer them if they presented a good investment from the outset, would they? They offer it because there's a high chance that without it, you wouldn't get anyone to let it. And, the developers often factor this into their asking price/profit margins anyway.
Getting back to your suggestion that most investors in UWS are Australian. I highly doubt this, because any Australian investor that has a reasonable knowledge in property investment wouldn't touch them with a barge pole, for the reasons I've listed above. Any who have I would suggest are rookies, are buying them as a live-in investment for their kids, or have received bad advice from agents or spruikers.
Ultimately the people who have commented here do have some experience in the local market. We're providing you advice because we want people to make the best investment choice
It's entirely up you whether you decide to go ahead or not, but please, do seriously consider what our thoughts are on this matter.