Can an employer and/or its employees be liable for harassment of an employee because of, for example, their partner's religion or belief?

Yes. The harassment may relate to the actual or perceived religion or belief of someone else such as an employee's partner or colleague. Such harassment may also be race discrimination. Another consequence of the harassment not having to be directed at the victim personally is that if, for example, an employee were to witness the religious harassment of another employee, this could create an offensive environment within the meaning of s.26 of the Act (which defines harassment) that could amount to harassment "related to" religion or belief, even though the unwanted conduct is directed towards a third party. The employer will be liable unless it has a reasonable steps defence. The individual employees will also be liable, whether or not the employer has a reasonable steps defence.