Halliday & Matthiessen (2014: 400):
The expression of voice is an extension of that of tense. The active has no explicit marker; the passive is expressed by be or get plus V-en (past/passive participle), appearing as an additional modifying element at the end. The passive thus functions like an additional secondary tense; and it displays a distinctive combination of presentness (be) and pastness (V-en) suggesting ‘to be in a present condition resulting from a past event’ … For this reason, there is no very clear line between passives and attributes having passive form.