It depends on whom you're flying with.
Qantas, for example, does not allow e-Cigarettes to be used on board.
The use of e-cigarettes is not permitted on any Qantas aircraft. If carried, they must only be packed in carry-on baggage, together with all spare batteries. Spare batteries must be individually protected so as to prevent short circuits using one of the following:
in original retail packaging;
taping over exposed terminals;
placing each battery in a separate plastic bag or protective pouch.
Neither does Emirates. Other airlines like Malaysian or Air New Zealand don't have documented policies. Your best bet is to ring whatever airline you're travelling with and check, but expect to be denied.