With Mighty/Britz at least, whether it's an imprint or a deposit depends on the insurance option chosen (imprint has higher rental cost/day), and they will refund the credit card surcharge for Visa & Mastercard.
http://www.britz.com.au/campervan-insurance-options
By traveller's reports, Apollo appears to just keep the surcharge and take their time to refund a deposit. Sounds like they will take an imprint for the other insurance option but not clear from their own terms. Their terms quote the cc surcharge, but they imply the the deposit is refunded on vehicle drop off.
http://www.apollocamper.com/terms/ap...1314-Quote.pdf
So I'm not surprised they've annoyed a few people. Not sure if the other companies are slow to refund the security deposit though.