Aw, you should come to Melbourne! I think Adelaide might be a bit warmer, but I'm not sure, as I've only been there once and it was in July (i.e it was really cold!). Here is a thing from last year (http://www.bom.gov.au/climate/current/month/vic/archive/201104.melbourne.shtml), to try and answer your question about what I define as a "lot cooler". Keep in mind it gets really hot over Summer, so April is like a dream!