The New Zealand Plant Conservation Network

Also found on the cliffs at Titahi Bay. Native or exotic?

Not sure - I am inclined to think this is glaucescent S. oleraceus - we had similar things on the Poor Knights in 1995, 1996 which we had to cultivate to see what they really were, and they changed their growth habit and leaf colouration to 'bog standard' S. oleraceus BUT you may have a more deeply toothed race of S. kirkii which was (when I was a kid anyway) common at Titahi Bay

I agree with Peter, more likely S. oleraceus as it has the leaves have the characteristic lobing of that species.

Astrid, if you are meaning Sonchus kirkii, you can see the real deal at Rocky Bay or at my place where it self-seeds everywhere. Have also recently found a plant in Pukerua Bay.