Working on a regular and systematic basis doesn’t require the employee to work every week or even on the same days. There only needs to be some sort of discernible regular pattern in a casual employee’s work hours.
Employers have been caught out before because they’ve failed to understand that regular doesn’t necessarily mean “frequent”.
For instance, a casual employee who is only rostered on the last Friday of every month works infrequently, but still has a regular and systematic work pattern.