Hello [USER=61]@Tumbleweed[/USER] , it's lovely to welcome you to our forum.
I love this plant, although it is more at home in New Zealand, its native land, we are fortunate indeed to have it here as well.
Looking at the plant you have there makes me think that your own instinct is right. Griselinia is prone to root rot. It enjoys a good old watering in dry summers, but too much, especially in soil that doesn't drain well which can be really bad news - something you'll see after a flood!
The bad news is, I think, that the affected plants are not very likely to survive if this is the case. When they grow to this stage it's not so easy to dig them out and replace their soil (not like a small plant in a pot)
If mine, I would now take some cuttings from the plants in the hedge that are not affected (they root quite easily) and if the ground does prove to be wet and not draining well, improve the situation with added grit or washed sharp sand for drainage. Good luck. Do stick around a while and let us know how you get on 
PS. My eyes are not as sharp as I'd like, so just to check, those are drops of water on the plant aren't they? Scale insects can also affect this plant.
