The Dictionary defines "airline" as:
1. Aeronautics.
a. a system furnishing air transport, usually scheduled, between specified points.
b. the airplanes, airports, etc., of such a system.
c. Often, airlines. a company that owns or operates such a system.
I am not aware of any CASA legislation which uses or defines the term "airline", therefore the dictionary definition wins in interpreting your contract!
The CASA definition of an operation you may term "airline" is contained within CAR 206 (1) (c), as tio540 pointed out, and does not use the term "airline".
The dictionary definition uses the term "usually scheduled" which tends to indicate the term "airline" is not particularly restricted to scheduled air services, as in the European use of "charter airline".