Brilliant one Alan [Linked Image]. Sadly it's all so believable.
Kiwi, it's a building code and both battery and hard wired mains detectors are acceptable. www.standards.org.au/downloads/060501_Smoke_Alarms_become_mandatory.pdf
and: http://www.nswfb.nsw.gov.au/community/athome/smokealarms/
something on the legislation here: http://www.fire.nsw.gov.au/community/factsheets/english/11a_smoke_alarm_legislation.php