1

I'll assume India is going to be your startling off point in Asia. India up to Nepal can certainly be done overland. Going overland from India to Thailand would probably mean travelling through Myanmar (Burma)....from what I heard this might be more hassle than its worth......someone with experience of travelling there would be in a position to give better advice. Anyway you could easily fly from India to SE Asia. Travelling overland through SE Asia down to Indonesia is easy to do and is a well trodden backpacker route. How many countries you want to stop off in depends on your timeframe. From SE Asia the best option to get to Oz is flying.....there are plenty of budget airlines doing that route like Jetstar.

2

You can't cross Myanmar overland.

If you start in India, in order to go Vietnam, you would need to make it through China. In order to go from India to China, you only have 2 options:

1) India-Nepal-China: This options means going through Tibet. Regulations for foreigners visiting Tibet change frequently and without prior notice. It could be closed for foreigners on your travel dates. When it isn't closed, you have to get a special permit to Tibet in addition to your Chinese visa.

2) India-Pakistan-China: With this option you skip Tibet. I didn't pay attention at your travel dates but the border between Pakistan and China is closed several months each year due to bad weather. Don't forget you have to get your visa for Pakistan in your home country. You would need to pay attention at the safety situation in the areas of Pakistan you would need to cross.

A budget flight from India (or Nepal) to Vietnam (e.g.: with Air Asia changing planes in a third airport) could be cheaper than making it overland (even if the cheapest flight option would require to stay overnight in the airport where you change planes).

Once in China, you can easily travel overland to Vietnam. Oveland travel among Vietnam, Cambodia, Laos, Thailand, peninsular Malaysia and Singapore it's easy. You can reach Indonesia by ferry from Singapore (I think that from Malaysia too). Once in Indonesia, you could make some island hopping by boat but it'll require some planning. Take a look at the web of Pelni to look for the schedule of domestic ferries.

There aren't scheduled ferries from Indonesia to Australia and you'll probably have to fly that leg of your trip.

3

If your heart is set on traveling overland between Asia and Australia there are some cruise ships that move between the two but there are typically only a few of these per year and are far more expensive than flying. (You may value the experience more than the money, cruises can be had for <$100 per day, and wish to pursue this option)

4

As anillos says above...if you want to do this to 'keep costs down'...it won't ;-)

If you want to do it because you love overland travel or because you want to avoid planes,that's another story.....

5

Indeed, travelling overland will, ironically, not be cheaper than flying. What it will be is more adventurous and a lot more fun. But basically you just need to decide if you want to travel in INdia, Nepal, Tibet, CHina or rather in SEAsia. If the latter than fly Delhi to Hanoi and then travel overland later.
Have a good trip.