Hi, I'm not sure where to put this question as it is very general nature - but I figure there must be some electricians floating around these parts :)

Is it safe to use power plug adapters long term? I don't mean plugging in an appliance that is only rated to work at 110v into an NZ outlet, but something that is meant to work at 240v and just uses a physical plug adapter to make it fit our power outlets.

I'd imagine it might has something to do with the load it is drawing and so far I've only used things <50 watts but I want to use a 500w item on a plug adapter and just want to make sure it is considered safe long term.

Thoughts? 

slight aside: I was going to use the word 'adaptor' but the spell check here likes to tell me it's spelled wrong?