When you sign the contract with your builder, you will notice there're a few clauses which talks about weather.
What it actually means is, if your builder finished your house on time, there's nothing to worry about. If you are unlucky and it wasn't on time due to whatever reason, the builder will get some figure about number of raining days, double it (every raining day requires extra day for the site to be dried), then use it as excuse for negotiation.
I briefly talked about this in my previous post [Tips.006] Ask for a copy of Contract in early stages.
This tips is about: instead of waiting for your builder to provide some high figure which you might not know how to verify, you might want to start collecting that data yourself. And it's quite simple, just visit this web site from Bureau of Meteorology.
As shown below, the web site shows the daily weather observation record of past 13 months for all areas:
I clicked on Sydney and this is what I got for the current month:
Scroll to the bottom and you can choose any record from previous 13 months. So all you have to do is check it once every few months and write down the total number of raining days. Or you might even want to print it out for record!
Hopefully everything is on time and you won't need to use this, but who knows, better be safe than sorry!