Work hours are a firm line written into the rules of all four countries, the U.S., Canada, the U.K. and Australia: going over by one hour violates the visa conditions

In plain words

All four countries set an upper limit on how many hours a student visa holder may work. Going over is a violation of the visa conditions, and you may have the visa canceled and be sent home, and later applications will also be questioned. The limits are: the U.S., 20 hours a week; the U.K., 20 hours a week for a degree course and 10 hours for below degree level; Canada, 24 hours a week; Australia, 48 hours per two weeks. If you hold two jobs, the hours are added together.