Can a Spouse witness a Postal Vote?
In most cases a Spouse can witness an Australia Election postal vote.
The only relevant condition appears to be that they are also eligible to vote, and are not a candidate in the election themselves.
Can a Husband witness a Postal Vote?
A Husband is a spouse, and the above answer applies.
Can a Wife witness a Postal Vote?
A Wife is a spouse, and the above answer applies.
The relevant wording at aec.gov.au is
Your witness needs to be enrolled to vote but cannot be a candidate in the election. A witness can be anyone who knows you, including a family member, a friend, or a neighbour. Your witness will make sure you fill out your own ballot papers – this is one of the ways the AEC makes sure that postal voting is secure.