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No. They should not lose their leaves, and if they do, it will signal that the environment is not quite what it should be.

Here is a document from the United States Geological Survey--Biological Resources Division
Haleakala Field Station, Maui, Hawai'i that will help:

http://hear.its.hawaii.edu/starr/hiplants/reports/pdf/carmona_retusa.pdf

Unfortunately, your photos did not come through. I am unable to see the damage, the lights, the soil, the type of bonsai, or any other information to help me to identify the issue.

Bonsai is, simply, a method of pruning. Problems will need to be addressed according the the genus and species of tree. Typical issues will be over/underwatering, lack of proper light spectrum, too much light, too little humidity, disease, nutrient deficiency, or many other issues. Unfortunately, I do not have enough information to give you any recommendation on what has gone wrong.

In the meantime, this collection of articles will help you with the art of bonsai and culture practices:

https://www.gardeningknowhow.com/houseplants/bonsai

I believe this is your bonsai tree. You can keep it indoors in winter and outside in summer. Here is the article:

https://www.gardeningknowhow.com/houseplants/ficus/how-to-grow-a-ficus-ginseng-bonsai-tree.htm

Since it is spring in Australia, your plant shouldn't be going into dormancy. It may be overwatered, so one thing you can do is repot it in fresh soil. Don't water for about a week. If the roots are damaged due to overwatering, they can't take up water and the plant looks dry even though you are watering.

I suggest reaching out to a bonsai society. Here is one to get you started: https://bonsai.asn.au/about-bsa/

https://www.gardeningknowhow.com/houseplants/bonsai/bonsai.htm