if you definitely know that the house is beyond saving why wait? starting now would allow for the demolition to be planned in a more organised way so that you can potentially salvage anything that might useful either in the new house or for recycling eg any windows, doors, water heaters etc that could be offered for sale or bricks that could be reused on site for feature walls or garden paving... on our last big reno we made several hundred dollars from salvaged electrical wiring, thousands from the sale of building materials and saved thousand in labour by cleaning some of the site ourselves and taking rubbish to the tip...not a fun job but cheaper than joining an exercise class ...luckily there was an old shed on site that we kept for builders' storage and we salvaged plants, had trees and shrubs trimmed and fences repaired before major new work began and all the extra time on site also gave us more time to feel the local climate, plan the new landscaping and meet the neighbours

Since you have bought a home that is unliveable, you might as well start now. Along with the excellent advice from @okloiuse, there could be challenges, that may come up after the home is demolished eg: soil type, site costs. These may not have been obvious when home is still in situ. Also, when we knocked down our old home to build, because we are near the river, council regulations required us to rat bait the old home for 4 weeks prior to demolition. Once you have a cleared site, you know exactly what your dealing with before building your new home. If you leave the demolition until just before build, then there may be unforeseen delays.

Also , I'm guessing the demo guys prefer summer too -- less rain , less mud , more light , more daylight hours .

Thank you all for the advice, much appreciated!

A recent Grand Designs Australia had construction delayed because asbestos was found in the soil under the old house and needed removal.

Is there anything about the block that makes demolition straight away a possible drawback, I don't know, is it steep and clearing it too early might make it vulnerable to erosion or something like that?