Perhaps I should have been a bit clearer. It is possible for one member of a party to cast all of the party votes while he is the only member of that party who is present in the chamber, but 75% of the rest of the members of that party must be within the parliamentary precinct; 25% of the members can be anywhere at all, outside of the precinct. The reasons they are away are a matter between them and their party.
It could be for anything at all. Quite often, it is to attend other business, outside of Wellington, particularly for ministers. It doesn't have to be for illness, child care, or any particular reason that is specified in any rules. It really is up to the party to agree that a member be absent.