This is a free collection that requires you to register with FamilySearch. The records contain over 619,000 births and baptisms in Australia ranging from the year 1792 through to 1981. NSW, South Australia, Tasmania, Victoria and Western Australia all have listings. Queensland however has none. This is not a complete collection of all Australia births and baptisms, it is however an excellent place to start.
The records provide a name, gender of the child, names of parents, date and place of christening, and date and place of birth. When you find the record you are seeking, select the document symbol in the results and a digital copy of the record will be displayed. These are commonly typed records found through parishes and stored at state archives.
You can conduct a search using first and last names, sex, birthplace and year.
FamilySearch houses many other Australian collections that are useful for tracing your family tree.