doubt there are a thing called undergrad dentistry..
i believe b den @usyd works similarly to mbbs @ usyd, whereby it's a graduate degree, ie you study a bacholar (any bacholar, at any uni), then u are eligible to apply...
it, imho, also has the flexibility of tapping into other field of study during ur undergrad yrs, so if some where along the track, u realise it's not the right thing for you, or the world has changed that ur degree is no longer "hot", you can always turn aroudn without much cost...