How to find Giga pty ltd contact details?

I have stupidly agreed to Giga pty ltd to allowing them to take domain name moey out of my account without writing any of their contact details! Now I want to cancel it, but cant find a way on line to contact them and bank is not allowed to cancel a direct debit payment! Help!!! Anyone know the contact details of Giga Pty Ltd I believe they are in Sydney NSW. Thanks in advance.