Casa don't care how much you private flying you do outside of your airline job as long as you don't exceed the CAO limitations for flight times.
Your airline however probably has a section in their FAM that says something along the lines of 'you must have permission to fly outside of the company as we own your 900hr a year limit'.
Exceptions to the 900hr rule are available however the request has to come from the company rather than from the pilot, or Casa tends not to grant them.
Most airlines seem to allow a basic figure like 10hrs or so a year without to much hassle but any more will require some bargaining with the boss, particularly if you are sailing close to the 900hr mark.