The justification for the North West Rail Link, contains a couple of interesting points in objective 4 in section 22.3.1:
(a) 29 million trips within 5 years of its opening, which is about half the number of trips currently using the bus system in the relevant region, region 4. Obviously, people will still continue to use buses into Parramatta and other locations but I expect more than half of bus users are going to or from the CBD-Macquarie Park area.
(b) almost 160 buses are to be removed from the CBD in the morning peak. This is approximately the amount of buses which enter the CBD in the morning peak from the North West.
I have little doubt that point (b) would not actually happen within the next decade, particularly while the rail-rail interchange at Chatswood remains.
Basically, the justification does not suggest an increase in the public transport usage in the North West.