Privet! I am not ill disposed to many plants but this is one I would like never to see. The name privet covers a number of different Ligustrum species. They tend to be a pest wherever they are introduced because they are so very successful. They produce squillions of little flowers that result in similar numbers of little purple fruit. Birds at least appreciate them but then go on to scatter the seeds in their droppings, and the resulting privet seedlings then out-compete the native plants and anything else, and take over. Privet is also poisonous to browsing animals, particularly horses. And the sweet cloying scent of the little flowers is thought to be the cause of the nasty allergic reactions experienced by lots of people - myself included.
At least the bumblebees enjoy the flowers. This one zoomed in as I took the photo, wings beating (and blurred) it paused for a quick feed then busily flew to another spray. Here the flowers are just opening so they are not at their most allergy-causing - I quickly shot some photos and made my escape, sneezes on their way.
Like gorse, another pest, privet was introduced to New Zealand as a hedging plant. The qualities that make them good hedges - robust, easy to grow, tolerant of a wide range of conditions and so on, are the very reason they were risky introductions - useful plants which turned into thugs.
There are a number of native plants in the genus Aciphylla that can be planted to defend territory because of a different thuggish defining characteristic - they draw blood. No kidding - some have spikes that can puncture tramping boots.
Don't mess with me say the formidable spines - here on top of a flowering spike, protecting the forming seeds, but the whole plant is defended by spines and sharpness.
Maybe the spines evolved to stop moa from eating the plants - there weren't any other browsing animals around before humans arrived. Apparently the roots and shoots of some Aciphylla are edible, but generally it is wise to give them a wide berth - falling into an Aciphylla is not recommended.