Have been doing some more research to try and get a better...

Have been doing some more research to try and get a better understanding of these damn NXXT trades and will summarise as follows.
They are in effect a special crossing which is a type of order where the buying and selling broker are the same. It appears that the terminology "off market crossing" really means that you don't have to sit in the market depth queue.
If the broker has two clients that want to buy/sell the same stock then the trade can take place either at or between the best bid and best ask prices. (NBBO) In some cases, where orders are a few rows down the list, the broker (Commsec) will cancel the order (remove it from the list) and match it with the counter order from other clients.
The volume must be reported on market. Someone as big as Commsec can effectively run their their own order book and when they have clients on both sides of the depth they can pull the orders in the list, pool the quantities and execute the trade as NXXT.
That ain't too bad for their clients as their orders can be filled without waiting their turn in the queue.
The term "non market moving" trades is probably spot on as they must be executed at or between the best bid and the best offer and therefore will not influence the price.
To settle at a price outside of the bid and ask the trade would need to be a true after hours off market trade.
It really has nothing to do with Chi-X at this stage as many of the stocks showing up as NXXT are not even listed on Chi-X. It is a Broker initiated trade.
The Centre Point crossing trades really are self explanatory. The price is the centre price between the bid and ask. A bid of 0.10 and an ask of 0.105 will see Centre Point trades going through at 0.102 (should be 0.1025 but they drop the last decimal)

Hope that all makes sense.
Cheers all
kse3137