It sounds like it is at the brokers discretion which trading venue they use.
Here's what Comsec says about the subject.
"What is Chi-X Australia?
Chi-X Australia is an alternative trading venue to ASX TradeMatch on the ASX which uses its own trading system and provides trading in a subset of ASX listed securities.
As we are connected to the Chi-X Australia trading venue, we may automatically send all or part of your order to this venue to be executed, seeking the best possible price, in accordance with our Best Execution Policy."
