Has there been a single case in human history where a farming people encountering hunter-gatherers in an environment suitable for farming has not rolled them up in short order?
Vickings vs Inuits in Greenland and North America. There were many causes for the Vikings failure, but Inuit pressure was one. (I concede, though, that the terrain isn't the best for farming)
Another case would be what happened on several occasions on Pre- and post- Columbian America. Before the Spaniards, in the River plate area, the Guaranni (recently arrived rudimentary farmers) were facing the hostility of tribes of hunter gatherers who had better cannoes and/or better bows. We don't know if the Guaranni would have prevailed at the end (I'd like to think they would have), but at the moment of the conquest they were having a hard time, due to the difficulties in adapting their "subtropical" crops to the new temperate environment (a problem the Maori might also have had).
Another example would be the struggle between sedentary societies slightly "civilized" on the Eastern side of the Andes (relatively far from the Andes proper) and "Amazonic" tribes from Chaco, who were mostly hunter gatherers (in OTL North Western Argentina and Southern Bolivia) Again, we don't know what would have been the final outcome. But according to Spanish accounts, the Amazonic tribes were beating sedentary societies hard.
Finally, there's also the case of the Amerindians who were agriculturers and, after the introduction of the horse, became Hunter Gatherer Nomads, and overrun those who remained agriculturers. Not the best example, as the aborigines wouldn't have had horses (Although maybe they could have fought mounted on cangaroos

)
Of course, we have to believe that agriculture would have triumphed in the end, if the enverinment were suited for it. In the South cone, for example, if the Europeans hadn't arrived, a few agricultural societies might have retreated for a while; but, since agriculture was firmly established in the Andes and in OTL Paraguay, it would have extended eventually, towards the whole Southern Cone.
But that doesn't mean than the agricultural societies who were living on the margins of the agricole world would
themselves have won their struggle for survival. When a rudimentary agricultural society gets into a new environment, dominated by a hunter gatherer society who has been on that terrain for ages and knows very well how to exploit the resources of the land, the victory of the newcommers isn't granted.
Translated to the Australian case, this means the folowing: if the Polynesians make persistant attemps to colonise Australia, they would probably succeed. But it's still possible that the first Maori settlers, surrounded by hostile aborigines and expiriencing difficulties with their crops (at first), find themselves overrun by Aborigines.
What happens nexts? Well, it all depends on whether their brothers back home chose to try it again (and again, and again), till they succeed.
(In any case, it's just an opinion. In the long run, I think the Maories might eventually settled in the East coast, merging perhaps with the Aborigines. By 1600, the East coast would have been mostly Maori in culture. The Aborigines would probably still be dominant in the interior.)