A credit limit of $20,000 is relatively high in Australia. For instance, if you applied for this limit on a card with an average interest rate of 19.94%, you would need to be able to afford monthly payments of around $753. This is based on the requirement for credit card providers to consider your ability to repay a balance, with interest over a 3-year period.
As another example, if you had a credit card with a $20,000 limit, it would typically affect your home loan borrowing power by $20k or more.