There's thunder in them there clouds - cumulonimbus cloud drama

The remnants of tropical cyclone Ita brought "severe" weather - wind and rain - to New Zealand at Easter time.  Wellington is used to storms and we didn't fare too badly.  We even had some most welcome very settled periods.  It was early evening - not much wind, quite mild - and I was pottering in the garden, focused on the ease of working the moist soil.  Then I heard "look at those clouds!"

Rolling in and building up rapidly before our eyes - pretty fluffy cumulus clouds turning sinister as they gained height and blocked the fading sunlight.

I'm shaky on cloud identification but my research suggests that they were forming into cumulonimbus capillatus clouds - tall with a raggedy top, very threatening and in fact about to produce lightning, thunder and rain.  I retreated.