I do possess a limited knowledge on this based on my experience at Australia and what I understand about other countries that you have mentioned.Climatic conditions in Australia is definitely a plus factor in its favour over Newzealand,Canada and to some extent U.S.,but job opportunities are severely limited in both Australia and Newzealand in comparison to Canada and U.S. and educational opportunities in Australia and Newzealand pales in significance to that available in U.S mainly and Canada to some extent.The only significant disadvantage with respect to Canada as I see is that your existing qualifications are not recognised for jobs,though jobs are available in plenty,as I understand,as the local experience is insisted upon.Well,unless the Canadian Government does something seriously about this significant disadvantage that the highly accomplished immigrants are confronted with,it is bound to certainly inhibit many prospective immigrants from migrating to Canada. The other major advantage of migrating to Canada is that it opens a door to proceed to U.S seme day which otherwise is extremely difficult to get into unless one proceeds to go to U.S on student visa.
Further,in the points system Canadian immigration policies are certainly more liberal than Australian or Newzealand's as the Canadian Government is willing to accept immigrants upto the age of 50.On the other hand the Australian and Newzealand Governments make it difficult for people over the age of 35 to migrate.
Well, as far as the job front is considered,IT is an exception in all these countries as the IT professionals do manage to settled down more easily than the others.