What it would cost to live in CM excluding rent ?

Roy -

As others have said, it all depends on your lifestyle.  I have lived in Chiang Mai for seven years and retired here three years ago.  I actually track every Baht I spend and prepare a report on my spending on a monthly basis. 

Transportation.  I suggest that you rent a car during your extended visit to Thailand.  Every single person I know who has a motor scooter has had an accident because the drivers here see motor scooters as an annoyance and tend to push them out of the way or simply don't see them.  A rental car plus fuel will cost 15-20K Baht per month.

Medical.  I don't know if your Australian medical insurance will cover you in Thailand.  Our American Medicare insurance does us no good overseas.  Medical care is quite inexpensive here in CM unless something major occurs.  A visit to a private hospital or clinic will cost a few hundred Baht per visit.  Most medications are available over the counter with the exception of sleeping pills which are prescription only.  I personally spend about 2K Baht per month on doctor's visits and medications, however, my wife spends much more because she sees specialists in Bangkok for her medical conditions.

Food.  We eat well and at good restaurants here rather than eating at food stalls.  A nice Italian, Vietnamese, German, Korean, or Western dinner for two usually costs us around 600 Baht including a beer.  I agree with the other response that says that you can eat for 30-60 Baht at a food stall but that decision is up to you.  Supermarket prices are at least 30% higher for name brands that you will recognize.  Local pork, chicken, and fresh vegetables are quite inexpensive in major supermarkets.  Beef is very expensive and the quality is lacking.  Wine is extremely expensive here.

Visas.  Depending on how long you plan to stay here, you might be able to get around the retirement visa hassle by making a border run when it comes time to renew your tourist visa.  Thailand is cracking down on this process but you could probably get away with it one time.

In summary, I agree with the other comments that you can live comfortably for about 50-60K per month plus rent.  Thailand is not quite the good deal that it is perceived to be.  In the end, it all depends on your life style and what your needs are.