A lot of Australian 7 pin plugs don't run the reversing lights on a trailer, at least not in the same way that a UK or European plug does. If they're the double thickness ones then they might have a feed to do so.
It's something I'm thinking about on my 4Runner as well, I could do with a bit more light sometimes reversing up our driveway (especially when I'm avoiding the tactical coloured motorbike...)
Pin 2 may be used for reverse lights in standard wiring configuration, but as Ed noted, many installers don't bother in Oz. Many in fact use it for a positive feed for batteries which I don't recommend.
But aside from that, if your wiring doesn't already have a reverse feed to that pin 2, then you will need to pick off the reverse light from the back of the light fitting.
I would not tap into the the reverse light as you may throw up problems with the canbus, you should be able to get a can bus friendly relay, A quick look on google and it seems the following relay will work and article;
10amp SY4050; http://www.autospeed.com/cms/article.html?&A=112936
i would double check with someone who is experienced with caravan installations.