2m sprouter

This 2m tall sapling shot up in our last remaining sunny spot through a tangle of Barleria and Leonotus. Thinking it was a Bridelia micrantha (Coast Goldenleaf - a lovely pioneer, but far too big for that spot and liable to shade the shrubs out) I reached in to try to pull it out. And was stabbed by a series of blunt 'spines' along the main stem. So I cut its head off. It's clearly no Bridelia. Suspect it's an alien. ID please!