In Australia, a crossed trade is legal only if it's executed at the current bid/sell level and only if there is a genuine market available. Which means, there has to be a bid/sell offer at the price the broker want's to do the crossing. If this isn't the case, then he can create this situation. Either way, the important thing is that it does happen at the current "Real" market value.
In the US it is illegal because they define it slightly different by assuming that a broker can do a trade without having to register it. In theory your broker could sell you shares another one of his clients wants to sell. He could do that at a price that isn't really reflecting the current market by lying to both parties and cashing in on the gap. This practice is also illegal in Australia so it all comes down to the definition.
Anyway, this leads to the conclusion that in fact a crossed trade on the ASX is a normal trade (in US terms) except for the fact that the buyer and seller is the same broker (but not the same client).
Ah, it's confusing, isn't it. Maybe somebody else has a better explaination. I'm off for the weekend.
Have a nice weeked
Stefan