There's a section in Mud and Gold when Frank faces near-disaster after several of his cows get into a patch of poisonous plants:
One of his cows lay on the ground, straining to get to her feet and moaning with the effort. Another was a few feet away, her unnaturally stiff limbs making it obvious she had been dead for some hours. Frank knew the cause even before he saw the scrubby plants around the edge of the clearing with their distinctive pattern of growth, each leaf directly opposite its pair on the long, thin branches instead of alternating up the stem: tutu, the bush farmer’s scourge, and perversely attractive to livestock.
Tutu (Coriaria arborea) is still fairly common in many parts of New Zealand. The eastern Bay of Plenty, where my fictional Ruatane is set, has traditionally had some large concentrations of this plant. Tutu is highly poisonous, and over the years it has caused many stock deaths, mostly in sheep and cattle. It's even been responsible for the deaths of two circus elephants which were inadvertently allowed to graze it, once in 1869 and again in 1957.
Humans, too, have felt the ill-effects of tutu, especially in the 19th century; sometimes attracted by the shiny berries, but more often from eating honey made by bees that have fed on tutu honeydew. My husband's grandfather fell dangerously ill as a young man from eating tutu-contaminated honey; fortunately he made a full recovery.
I rarely see tutu these days unless I'm travelling further afield, as it doesn't seem to grow well in this area (which is probably for the best!), but I did come across a patch on a recent outing. Here's a flourishing plant:
Note the distinctive growth pattern, with the deeply-veined leaves set opposite each other on the stems. Those long trailing stalks will have flowers later in the season, followed by invitingly shiny dark berries.
Maori skilled in plant lore used to make a delicacy from tutu berries, after carefully straining out the toxic seeds, but it's not something to be attempted lightly.
Frank learned a hard lesson in the vital importance of maintaining fences; a lesson he never forgot.