There are several unreferenced sources online saying Dianella nigra berries are toxic to humans despite being popular with birds. According to the book "The Poisonous Plants in New Zealand", by Connor, the berries were suspected of causing the death of a child in the nineteenth century but there was no evidence the berries were the cause. This is probably the origin of the unlikely theory they are toxic. Connor goes on to say there is no evidence they are toxic.
Low (1) says all (Australian) Dianella that taste good are safe to eat in small amounts, he does not actually say large amounts are harmful, he may just be playing it safe.
I have been eating the purple berries of what I'm pretty certain is Dianella nigra planted at the parking lot of Appleton Park, Karori, Wellington, New Zealand. They are rather insipid, but of good juicy texture and the seeds have a crunch. I find them reasonably good if you count the interesting color, acceptable added to a salad, more of a small vegetable than a fruit as they are not sweet. I think worth having in the garden as a salad backup, especially for a shady spot. Left to dry and shrivel they resemble raisons in texture. I think one of the better native berries to eat, admittedly that's not saying much.
Interesting Crowe (2) says they are not known to be toxic but are unpleasantly bitter, I did not find them to be bitter at all. Perhaps I have been eating an edible Australian Dianella but this seems unlikely (see photos of what I've been eating below).
It seems more likely there is variation in taste of berries, recently Dianella nigra was divided into three distinct species that can interbreed (3), Crowe may have sampled a different species or strain to me, the berries can vary in colour so variation in taste seems possible.
References.
1 Wild Food Plants of Australia. T Low.
2) A Field Guide to the Edible Native Plants of New Zealand. A Crowe
3) https://www.oratianatives.co.nz/catalogue_extras.php?article_id=215&catalogue_id=305
