Years on, these sea slugs have popped up on my radar again. I spotted them in Manta Bay and on Magic Wall at the Poor Knights Islands on 24-25 September 2016. This specimen is from Danger Rock outside of the Bay of Islands (9 October 2016). It was in a depth of about 14-16 metres.
Runcina katipoides is a sea slug but not a nudibranch. It is truly tiny; the slugs I've seen have been about 2-3mm long. It is probably more common than you might believe but its diminutive size makes it difficult to spot unless you're really trying to find micro molluscs.
This photo was taken with my Canon 5D Mark II using a 100mm macro lens. The photograph has been heavily cropped so that you can see the sea slug.