Double adaptors were in common use a decade or so ago, but are becoming increasingly less common. There are some very good reasons for this!
Double adaptors cannot be used on building sites in Victoria, but there is no legislation that prevents them from being used in other workplaces or homes.
By nature of their design, and as a result of the habits people have developed using them, they can be extremely dangerous.
Double adaptors rarely have any form of overload or surge protection to limit the amount of current drawn through them, in fact here at Jez Test and Tag we have never seen one that has.
They can be prone to overheating due to increased electrical load, particularly when they are piggybacked on top of each other (which you should NEVER do). This load can significantly increase the risk of overheating and fire.
Their design also lends double adaptors to being easily knocked and dislodged from sockets, or other adaptors if piggybacked. This can create a major hazard by introducing increased risk of electrocution resulting from someone touching the partially exposed pins. The majority of double adaptors in homes are pre-2005, so very few have insulated active and neutral pins.
Jez Test and Tag does not endorse the use of double adaptors, and recommends they be replaced with powerboards or the addition of more electrical outlets (wall sockets). Double adaptors are not covered by AS/NZS 3760:2010, and consequently are not tested for electrical safe.