The temperate latitudes of the Indian Ocean and adjacent regions are essentially empty- only the extreme Southwest of Australia and extreme South of Africa dip into the temperate belt.
What if there actually was significant land there in the 40s South?
There are essentially two options:
I- The "Realistic" Option: The Kerguelen LIP (perhaps including the Broken Ridge and Ile de Amsterdam still affixed to the rest of the plateau) is still very volcanically active for some reason, and an area over twice the size of Britain, and perhaps larger than France is above the water from latitudes 43 S to 53 S (parts of the Kerguelen Plateau were subaerial IRL as recently as 20 million years ago).
Depicted: the Kerguelen LIP.
II- Complete ASB Option: There is a large continent, possibly the size of India or a bit larger in the same general area, stretching from 40 S down into the 50s S.
The main difference between these options is the climate that they imply (lets assume that there will be no impacts on the rest of the world, or else it is a moot point to discuss whether or not European colonizers could have reached these places).
In the first, "realistic" case, geologically speaking, the land is like Iceland, comprised mostly of basalt and crowned with broad volcanoes that reach as high as 2.75 km. The climate would likely be cool- but warmer than IRL Kerguelen. Still, in the north it would be marine with cool summers and near but likely above freezing winters (this could easily support forests such as those on the South Island and in Valdivia). Further south, of course, temperatures would slowly become cooler until the Southern edge would have a climate like the actual Heard and McDonald islands- tundra/ice.
In the second, its geology is indeterminate as it is not based on the Earth's actual history. Its climate though would be more seasonal and therefore warmer in the summer, though with potential for harsh, continental winters especially in the interior and south. At the far southern edges, the climate would still likely transition to tundra as the land is deeper into Antarctic waters.
In both cases, the climate will likely have ample, frequent rain and be exceptionally cloudy.
Many may be inclined to dismiss settlement on either of these hypothetical landmasses entirely, but that is overly hasty. As the land is larger now, only the coast is scoured by the constant westerly circumpolar winds. Owing to their size, both would be able to support forests in at least a portion of their land (geological expeditions to IRL Kerguelen have revealed that when the plateau was exposed, it was home to forests, as evidenced by fossilized wood). In addition, the fishing and whaling on the west coast of either land mass would likely be some of the best in the world, and again, owing to larger size, increased continentality, and a lower latitude at up to 40/43 S, summer temperatures would generally be warmer than at comparable latitudes in IRL Kerguelen). Also, though this more minor, Kerguelen is home to a species of "cabbage" which is a useful source of Vitamin C.
Settlement and Main Issue
So, with the land and climate in mind, is it at all possible that a European colony founded in one of these scenarios could become more populous and therefore, more economically important and powerful than Australia did in our own world? Is it possible that a subaerial Kerguelen LIP could be like a Southern Britian, or a large southern continent perhaps even a 2nd USA or Brazil?
As for who would settle there it is uncertain, but the landmass could be hypothetically discovered as early as the 1520s and would almost certainly have been found and realized to be a large landmass by the 1640s. The Basque or Portuguese, later Dutch settlers, or the British and Irish are all possibilities.
A question sort of implied by this one is why did Australia receive fewer immigrants than the United States? In the latter decades of the 19th Century alone, the US received more immigrants than the entire present day population of Australia.
If this is due to proximity to Europe, then I think an Indian Ocean great power or even regional great power would be out of the question. But if it is due to the lack of Arable land in Australia, maybe there is a chance. Or perhaps there is another factor, social or political that motivated the disparity between the two. This would have a large impact on whether or not these landmasses would have a future beyond seasonal whaling bases or sparsely settled sheep stations.
Thanks for your input