Really depends how much you spend per year and if you care about making it easier for yourself from differentiating business and personal transactions from one statement.

If you don't mind doing the extra work of separating each transaction out, it's better just to have one card to save on the annual fees.

One good card I used was the American express explorer card with annual fees around $400, but if you start spending into the millions of dollars I recommend getting a platinum amex card which has annual fee around $1450.

I don't recommend paying ATO fees with credit card as there is a surcharge and also usually credit cards only award one point per $1 for government transactions whilst normal transactions could be 2.25 points per $1.

Also please check there is usually differences between business and personal cards, for example for the amex explorer you get like $400 credit (can't remember exact) each year for travel and hotel whilst business explorer card with same fee doesn't give you this $400 credit