As Drewboy says, a smart solenoid ((Voltage Sensitive Relay-VSR) is the way to go. It allows the 2nd battery to be charged but prevents the starter battery being discharged. Unfortunately you just missed someone selling one here - I think it went (used) for $40 or $50. There are other ways, but this is the best fit & forget budget option.
http://www.ebay.com.au/itm/REDARC-SB...item460a769ead
You can get one a bit cheaper on from other sellers on Ebay, but if you buy from a non authorised Redarc reseller you will have no warranty. This seller is an authorised reseller.