Hi Graham
With the humidity in Brisbane, I find that I have to bake my SBIG Desiccant Plugs (ST7E & ST2000XM) more or less annually to avoid frosting in the sensor chamber.
It involves unscrewing the Desiccant Plug (housing) from the rear of the camera body and placing it in an oven for 3 to 4 hours at a controlled temperature (<175 Deg C), to avoid melting the solder used in the fabrication of the housing.
This is what the Desiccant Plug looks like, approx. ¾ to 1 inch long.
http://www.optcorp.com/sbig-sb-50141-dummy-desiccant-plug-replacement.html
Cheers
Dennis