An employer is entitled to access your inbox or voicemail if they believe that there will be business communications that need to be dealt with in your absence, and if they have told you (for example in an internet policy) that this will happen whilst you are away.
Employers are also entitled to do this if they suspect serious wrongdoing such as fraud.
Employers should do their best to avoid accessing communications that can be positively identified as not relating to the business without opening them (e.g. emails marked 'Personal' in the subject line).
To be more assured of privacy in such a situation, it is advisable to start the subject lines of any personal emails with the word 'Personal' and any confidential emails relating to union matters with the word 'Union'.
However, bear in mind that this will not guarantee any level of privacy if the employer has another reason to intercept your email (e.g. if they suspect criminal activity, or they need to check that the business is complying with legal obligations).
To minimise risk of your private or union emails being intercepted and read, it is best to avoid using a work email or voicemail system for private or union matters.