If your car battery is old and what I mean by old is greater than two (2) years old, it may fail very soon. Most batteries for 4WD vehicles or Toyota 4x4 vehicles usually last 2 years in the Top End of the Northern Territory. The battery terminal may have blue powder or corrosion, this can be caused by a number of things, firstly, the terminal connector might be loose, causing the electricituy to arc between power supply and connector. Secondly, the battery fluid is low, this may need a top up. Read battery instructions and work out what fluid is needed for a top up.
Finally, the battery could be old they usually last about 2 years in Darwin. Have a look at the photo you can see the blue powder this is not good. Make sure you check your battery before you go on a long bush trip. Doing a 4WD course will help you