As far as I know BAS is simple a statement about GST received and paid by business, easily done by business accountant.
Audited reports are authentic and trustworthy financial statements prepared and signed by accountant. Immi ask employer to provide current and precious Balance sheets and profits& loss statements so they can see the progress of business.
These docs will be prepared by business accountant. He also can provide analyzed report which shows progress e.g sale, profit etc. These r common docs asked by immi.