Generally speaking, the main reason that visa applications are refused is due to applicants submitting the application with incomplete information. This is where using a professional service and Registered Migration Agent (such as Visa Australia) can save you time and money by ensuring all your information lodged with your application is correct.
There are many other reasons why Australian visa applications get refused including:
providing false and incorrect information;
missing proof of financial information;
nominating the wrong occupation from the Skilled Occupations List (SOL);
using a tourist visa to visit relatives or partners;
not meeting the strict health requirements that Australia upholds; and
not providing proof that you are competent in the English language.