Yeah, the blow fly works well in the Marlborough High country, I was up there a month or so back and was using a large damsel fly imitator, and was getting strikes. A couple of other guys were using blow fly imitations and was also having luck. At this time of year, if it looks like a insect a trout would probably have a go at it… Although, I mostly only catch small-medium size brown trout on Terrestrials, the bigger ones seem more inclined to take Nymphs (or streamers/lures).
I am normally only try fishing large terrestrials after I hear the first Cicada, before that its either nymphs or streamers.
Recently purchased a dozen Mouse flys from Flyshop.co.nz for $18, they have very minor rust on the hooks so were heavily discounted. many had no rust. They are normally over $5 each so was very good buying. Tempted to try flicking one of them around like a Cicada fly, see if any trout mistake it for a oversize Cicada.