It's related to the Chi-X trading platform which started in late November last year. In the past, when all transactions were conducted on the ASX, there was transparency for all participants as to the Bid/Ask price. Now, with multiple trading platforms, the ASX has set up a new NBBO (National Best Bid & Offer) platform called ASX-PureMatch. Basically, when bid/offers are submitted to either the ASX or Chi-X exchange, they go into the PureMatch trading system and the are executed as if they were all part of one, big trading system.
From a reporting code point of view, however, if one side of the trade is on the traditional ASX system and the other side is on Chi-X, it is reported as NXXT.
The reason why you are starting see more of it is because Chi-X only started out with the Top 50 companies, but has gradually been expanding their coverage.
MCO Price at posting:
23.0¢ Sentiment: LT Buy Disclosure: Held