Howaye LeeWT,
It was in the 40's when I was there in Aug and to be honest I wouldn't fancy swingin' a pick in that heat!
By and large I found it ok because everywhere has air con.
I will be working in construction so I visited one of the company's sites and it was seriously hot - the lads on site were wet thru' so I'm sure it takes its toll on the body. That said the site offices were all air conditioned.
If you are thinking of going over in Dec / Jan, then you will have some chance to acclimitase but be prepared for June - Sept.