To get back to the original question, the Australian DC-9 fleets were built up, side-by-side, in the 1960s-70s era of the Two-Airline policy between TAA and Ansett. The DC-9 came in 1967 when the 737 was not around and other options, such as the One-Eleven, not what was required.
The rollover by Ansett from the DC-9 to the 737-200 was the result of steady Boeing sales pressure, they had more than one serious go at both carriers (you find an earlier attempt documented in Brogden's book on the Two-Airline policy). The difference was marginal, the two aircraft were fairly comparable, in the end one went for the trade-in and one didn't. You could probably draw up a long list of the advantages both of doing it and not doing it. Boeing passed the DC-9s on to a dealer who disposed of them quitequickly in the USA.