There are two aspects to this. First, whether your employer has a right to film you without your consent (although check your contract of employment, you may find you have consented to any monitoring or recording as a condition). The answer is basically "yes". As they are the property owner they can film anyone they wish on their premises, except in circumstances where there is a reasonable right to expect privacy. This would preclude them filming you in a toilet or restroom, perhaps even a staff canteen, but not otherwise. Almost every mall and shop uses CCTV without asking permission, and this is basically the same (CCTV has a legal requirement for a notice informing people of the monitoring, but this does not apply to ad hoc photography).

The second part is whether the employer can use the film as evidence. I can't think of any reason why not.