Yup, I agree with Warrenk. I too am a licenced electrician.
The design of the 10 amp, 15 amp, 20 amp, 25 amp, and 32 amp single phase outlets is that the small can go into the bigger, but not the other way around. The physical interlock is the size and shape of the earth pin that increases with size. The 10 amp plug can go into (and open the shutters) of the 15 amp.
See
https://en.wikipedia.org/wiki/AS/NZS_3112 for pictures.
Not sure why DETA decided to depart from a very long standing practice.
Those arguing about the upstream protection size are barking up the wrong tree here as well. Most domestic power circuit are either protected by a 16 amp fuse or a 20 amp circuit breaker. (And, these days, hopefully everything is also covered by an RCD.) If one looks at the operating characteristic of both, in the overload part of the curve, they are about the same. The 20A circuit breaker is much, much, better in clearing short circuits than the 16 amp fuse.
When putting a 10A plug into a 15A socket, thermal overload is not a problem as the device is limited to under 10A. The only concern is having sufficient low impedance as to clear a fault using the overcurrent part of the protective curve. Given the size of the conductor in the appliance, this is not a concern either as the cable fault rating for 1mm2 flexible is 1.2x104 A²s
There are a few options ...
1) Depending on the size of the wiring and the selected circuit protection to the existing 15A outlets (e.g. 4mm cable protected by a 20A CB per 15A outlet) it is permissible under Table C9 (page 484 of AS3000) to have up to eight additional 10A socket-outlets in that circuit (under certain circumstances).
2) There are legit 15A socket-outlet to 10A plug adapters that incorporate a miniature 10A CB. This might be a cheaper option than getting a sparkie out to fix the existing 15A outlet or installing a 10A beside it.