Looking for a good location in MEL /4-5 days & SYD /4-5 days...

Guidance on centric hotel location- MEL & SYD

 also where to get a 3day 'getaway' to Cairns. thx  

SYD: Circular Quay (best location, gorgeous views)

In Cairns I'd suggest in town somewhere closer to the marina, for good options for dining and going out to the reef. In this area are Shangri-La, Hilton, also Pullman and Pacific International, among many others. 

I enjoy the Rydges Melbourne on Exhibition Street... big rooms, central location which is walkable everywhere in the CBD

I'm a big sucker of the Park Hyatt in Melbourne, nicer than the Grand, great restaurant, and closer to my favourite South American restaurant.. 

Sydney I tend to stay at either the Westin on Martin Place or Radisson on O'Connell. Both are right between Pitt St mall and Circular Quay. Easy walk to either.

Truly depends what you want? I'm more one for the smaller boutique hotels:

Piedmonde Apartments in Cairns  excellent location , great views and close to Oh Cha Cha a very good Japanese Restaurant a two minutes  walk away