Depends on where and when, of course.
In Australia 40 years ago: almost never intermarried. In Australia now: often do intermarry; intermarriage exceeded 50% some time in the last ten years.
In Greece a century ago: almost never intermarried. There weren’t a lot of non-Greeks around to marry (depending on your definition of non-Greek, of course; I’m taking an expansive one). Now: less so.
Of course, that answer is a commonplace. More concretely: Greeks are by default endogamous in diasporas: they are rather attached to maintaining their cultural identity in the face of what they see as an external threat. Unusually so, compared with other migrant groups.
That gets mitigated by various interrelated factors.
- Size of settlement: small Maniat settlements in Italy in the 17th century assimilated rather readily.
- Local authority figures: the colony in Corsica did not assimilate in the critical first two generations, because they had a monasteryful of monks and several chieftains with them, urging them to stay Greek Orthodox.
- Time: the second generation of Greek Australians didn’t intermarry; the third did.
- Sense of threat: the Greeks that migrated from Corsica and Mani to New Smyrna Beach, Florida were a minority of the settlers; the majority were Minorcans (especially once malaria got the Maniots). In New Smyrna, it was the Minorcans and the Greco-Corsicans against the cruelty of Andrew Turnbull. In Corsica, the Greeks were at constant war with the Catholic Corsicans for another three generations; in Florida, the same Greeks intermarried with the Minorcans immediately. The Minorcans weren’t the threat any more; the English were.