Reading the info on my voucher for my booking in Darwin, it say I require a credit card on picking up the vehicle to hold the security deposit but I never use a Credit Card.
OK. Do you have a credit card?
Yes, I have a Debit Card
Yes it is okay, if you read fully your voucher:-
This location accepts Debit Cards. Conditions of debit card use: card must be in main driver's name. MasterCard or VISA only. Debit cards must be issued by a bank, and numbers must be embossed. Please note, the deposit amount will be charged if using a debit card and associated transactions fees may apply. Maximum rental duration 28 days if using a debit card. Prepaid and electronic cards (Visa Electron) are not accepted.