The migration officer will already know about your financial position from reading your documents, and you do not need to impress anyone. Just wear your tidiest casual clothes. If you are a business man, then by all means wear a suit, especially if you have applied for migration under one of the business skills sections. If you are a professional, then wear whatever is normal tidy wear for your profession. For someone in trade, wear smart casual clothes.
Ladies have an easier choice.
Put on something smart, but not too sexy. You want to look like the migration officer's sister. If you are bringing children with you, they should have clean, tidy clothes. If you are coming from a country where the clothing is different from everyday wear in Australia, then wear your usual costume. Wear it with pride.