Hi, In my opinion among many eCommerce platforms available, the best are
1. Shopify
2. WooCommerce
3. Magento
4. BigCommerce
5. Wix eCommerce
If you have a small or medium-sized business and if you do not have a website yet then I would suggest you Shopify. It provides a full package of features which is required to build and run a successful eCommerce store.
If you are already having a WordPress website and want to just start an online store then I would recommend WooCommerce.
If you are looking for a well-reputed company in Australia for eCommerce development then I recommend you to consult Fortunesoft IT Innovations ( https://www.fortunesoftit.com/au/ecommerce-development-company-australia/

We have been selling online for around twelve years. Our main store was migrated to Shopify around 7 years ago - no complaints, easy to use, great plugins, plenty of people available to outsource tasks to if you are not comfy and great integration to shopping feeds. Only negative would be cost, but that negligible if what you are selling easily covers the monthly fee. Fee is approx US$75 per month.

We have recently spun up a couple of new eCommerce sites testing the waters with other products and solutions. For that we use AWS instances hosted in Sydney running WooCommerce. WooCommerce is also a great solution but takes a bit more technical work to setup and maintain. This can be a lot cheaper, we have one site running full-time which is a t3.micro on a reserved instance which costs around AU$70 a year.

Hope that helps.

Thanks @Allisha Martin ... Some others are Salehoo, , Oberlo, AliBaba, AliExpress... Some tips in this article here...  https://www.savvysme.com.au/article/1961-eCommerce-tips--tricks

Shopify is ideal for starting out and can let you scale to a great size. However, really depends on what sort of customer Journey you want to provide and the scale of what you're selling. Shopfiy can be extended to run as a headless e-commerce platform or there are other solutions to provide a better user experience.