interest rates, how low can they go?

This topic may have already been covered but anyway.

It may seem a bit of a simple question, what is the lowest rate a bank would lend money at? If the RBA was to keep dropping the cash rate to zero at what point is the interest rate offered to borrowers simply a reflection of the banks profit margin. I realise it would then be determined largely by where the bank was getting its funds from.

I know its a bit of a theoretical question but what are the brokers thoughts on this.?