Intergrades are not crosses of anything - they are a local colour form of carpet, and fill the space between where diamonds finish and coastal carpets begin. They are just a natural gradation that occurs on a particular part of the coast. There is no evidence that diamonds and coastals existed as separate entities in the south and north, and blended in the middle at some later time. They have probably been living in their range as long as coastals and diamonds have been in theirs.
I'm with Duga on this one tho, I'd say hybrid, but I have seen very similar "coastals" around Coffs Harbour.
Jamie.