Actually, you could be.
In many cases, esp along Orchard Rd, the private/public property boundaries extend beyond the buildings themselves, sometimes well beyond. Take Ngee Ann City, for example -- the large area around the fountain is private property.
The boundaries are not always clear. I did a lot of the planning for a Starbucks Christmas Open House promo in 2007, and had the 'pleasure' of applying for permits to place 8 giant cups along Orchard (you may remember them -- they were a popular spot for photos
). SLA (Singapore Land Authority) maintains the records, and I heard that the boundaries change every now and then. I even found a little spot of land that SLA said was not public land, but which the building management said wasn't theirs either -- go figure!
Now, if you're just shooting for yourself, dealing with the SLA may be more pain than you're willing to go through. Dealing with building management may not be much fun either -- some are better than others, but bearing in mind you aren't commissioned by a tenant, have no government affiliation and are not acting on behalf of a registered charity (all of which helped me!), you may not get much joy.
Shooting from public land is probably far less bothersome, but beware of making too much of a nuisance and causing any obstruction to pedestrian traffic -- if there's a complaint, your shoot may be interrupted by the cops.
I'd suggest that if security stops you, ask them where their property ends, and pull back to there -- as far as I remember, few of them own the land all the way to the walkways adjacent to the road.
Or be sneaky: ask them if you can talk to the management, send one of your party in, and quickly finish your shoot.
In either case, plan your shoot well and execute quickly -- don't linger.