Are there other reason for the loss of the megafauna...

The main reasons as to how we lost the megafauna are as follows:

Human Impact: We needed a lot of land to support our lifestyles over the years, so many forest got deforest. None of the forest revived so native animals and plants that live there lost their habitat. The others are hunting or excessive burning off in the grass lands.
One of the biggest causes of the extinction of megafauna was because of when the humans arrived around 48,000-60,000 years ago.

Aboroginal burning: They burnt the land for a whole range of reasons. They burnt the land so that they can move across it easily. They hunt along the fire front. They signal distant bands. Fire promotes the growth of beneficial plants that sprout after burning. So there are many reasons why humans might be burning the landscape in a way that they thought would have a positive impact. The Australian landscape is unique in that it has extremely low nutrients in the soils, because it's a very ancient, flat landscape, and most of the nutrients have already been removed from the soil. So it has a high sensitivity to burning having a big impact. Large animals in the interior of the continent were eating just such a mixture—feasting on nutritious grasses in years with good rains, and depending on trees and shrubs in drier years. After 45,000 years ago, those nutritious grasses were nearly gone, and those animals that hadn't gone extinct were restricted to dominantly tree and shrub dietary sources.

Natural Disasters: There are theories that natural disasters like a severe drought or blizzard killed the megafauna. A volcanic eruption is one of the theories too.

Mass Diseases: Diseases, infections or viruses could have swept Australia killing all the megafauna.

Change in Temperature: There could have been a big shift in temperature making it too hot or too cold for the megafauna to survive.

Invasion by non-native plants/animals. Other plants/animals are brought to Australia for certain reasons. Species that are already living there wouldn’t get used to the them. This can cause more problems.

how the climate change over time in Australia......

When Australia broke off from Antarctica maybe 50 or 60 million years ago, it was at a high southern latitude, where conditions were usually moist and cool. It had forest over the whole country, so it had a very damp, cool and forested land. Then it slowly drifted north towards Asia, and in that process, it moved from those temperate latitudes to the subtropical high-pressure zone, where the continent became both warmer and drier. Therefore, the inner of the continent in particular dried out. And the fossil record shows the vegetation and the fauna adapting to those much more dry conditions.

Superimposed on the long-term shift from that a cool, moist climate into subtropical aridity, the world entered an ice age about two million years ago with glacial/interglacial cycles. Consequently, the last two million years of that 50 million-year journey was a period of high-amplitude climate change, from relatively warm, wet climates to much drier and colder climates, back and forth repeatedly. And these transitions occurred rapidly, many times over a few hundred to a few thousand years. This would have been a very different kind of climate stressor from the more gradual shift towards aridity.

Credit: http://www.pbs.org/wgbh/nova/bonediggers/extinction.html

Could the plants have played a role in the extinction of megafauna?

If the plants that the animal eats go extinct, then the animal would have no food to eat, and eventually they will die out. Therefore, the food chain is disrupted and this could be cause of the extinction of megafauna. The same effect goes for today, if vegetation dies out, the same scene will appear again.

Did climate change affect the extinction rate of the megafauna?

Did climate change affect the extinction rate of the megafauna?

Maybe, maybe not. We have no certain data to prove or disprove this theory, it is to early for us to know. Most of the people we interviewed said that climate change could have been a reason for the extinction rate of megafauna. Also we must remember that these animals had survived 2 million years on earth, and would have endured any previous climate change that occurred.

There's no clear evidence that the ice-age cycles themselves led to any of the big extinctions.

Tribute To Extinct Australian Megafauna

What is Megafauna?

Have you ever seen a giant wombat in Australia? Of course not because they don't exist anymore. But once upon a time they did, along with other giant animals.

Megafauna literally means BIG animal.
Mega + Fauna = Big + Animals. Any mammal weighing more than 100 pounds (45 kilograms) are considered as megafauna. Excluding dinosaurs however because they're in a category of their own. Long after the extinction of dinosaurs, megafauna roamed Australia. Giant flightless birds, hippo-sized diprotodonts and the world's largest ever land-dwelling lizards make up the megafauna, which went extinct around 30-50,000 years ago.