I want to place a country in Oceania, but its eastern coasts appear to be too hot in summers, I want to change its climate a little bit to make it more livable in summer, how can we change its geography to bring such changes to about 5 to 10 degrees C° lower especially south of real life Brisbane.
You could really consider anything south of Sydney to have the temperate weather zones you are after. Keeping in mind though the lower the humidity will be the greater the max/min variants (Melbourne will commonly have dry heats of over 40 in summer and freezing lows in winter).
Maybe New Zealand/Tasmanian equivalents would make a good choice?
$\begingroup$ New Zealand's East Coasts are indeed 5 to 10 degrees colder than Australia's, even at the same latitude. Australia has a weird near-permanent giant anticyclone that keeps it hot and dry. Any other country down under probably won't. A smaller landmass like New Zealand or Tasmania definitely won't. $\endgroup$ Oct 3, 2022 at 9:08