Thanks for the feedback. It can be annoying if you are at the...

Thanks for the feedback.
It can be annoying if you are at the top of the queue in a thinly traded stock, and there are Chi-X sales.
My rationale was that it may be worthwhile to have an account with a broker that uses Chi-X.
Place 2 orders (one ASX and the other Chi-X), thereby increasing the chances of filling an order (perhaps at a slightly higher brokerage cost).
There can also be Chi-X crossing prior to ASX market open.

However, given that your broker is Commsec, that raises more questions than answers.
Whilst I didn't (couldn't) check, I doubt that there wasn't any ASX-market buy orders in the queue.

The sales were NOT crossings (XT), so, based on my understanding (which could be wrong), we know that Commsec, your broker, was the seller, and that a different broker was the buyer.

From my Commsec orders, it has always (at leaset those that I checked) been ASX as the primary market.

One possible explanation (and it is only a [wishful] guess) is that there is a second-Commsec-broker-entity, that uses Chi-X as its primary exchange.
Perhaps an institutional-focused broker. Chi-X may have better rates for larger orders (institutional?) clients.
This may imply (this is where the wishful thinking kicks in) that the buyer was a larger player, or possibly an institution.
.
Unfortunately, my primary objective of joining the Chi-X and ASX queues has not made any headway.
Again, thanks for the feedback.