The link gives you a full rundown on its history.
I can't say anything that hasn't already been said but it's a must see! I personally (from a stonemasons perspective) find the construction of the actual house breathtaking!
You are looking for a large pipe hidden in the bushes! The challenge of this one is to get in and out without getting over run with bull ants! The cache is well worth the effort! The map isnt entirly clear but a dirt road alows you to drive up to within 6m of the GZ! Enjoy...and as usual, NO SNOWMOBILES!