Fares are based on how many zones are travelled through in a journey, and route 77 travels through zone 1 when it goes through the CLEM7 tunnel (see below).
For example, a journey from Buranda to Windsor station would be a two zone fare as you would travel from zone 2 (Buranda) through zone 1 via the CLEM7 and finish again in zone 2 (Windsor station).
Route 77 map, click to enlarge.