I wasn't actually watching depth, but looking the trades yesterday, I'd say it was a "big sell" into existing buyers rather than a "big buy" - since 13.5cps was the low for the day and the trade took the price down from 14cps.
My best guess is that this is Orbis selling down. They have already filed two SSH notices showing selling and will have to file for every 5m or so that they sell until they get below 5%. It has looked as though they have been continuing to sell into depth, first at 14.5cps, then 14cps and perhaps now at 13.5cps.
Of course, for every share sold there is someone prepared to buy - though it is possible some may just be covering a short.
NBS Price at posting:
13.5¢ Sentiment: Sell Disclosure: Not Held