That's probably just because Thedas follows the monthly calendar developed by the Chantry; and before that, Tevinter.
The only reason the Earth Northern Hemisphere Spring starts in March is because the Gregorian calendar was developed by the Catholic church in Italy, based on the Julian calendar developed by Rome - both in the Northern Hemisphere.
If the 'standard' calendar had been developed in Australia or Argentina, I'd imagine New Years would be July 1st.
Not necessarily, In fact as Australia doesn't have harsh winters life doesn't "stop" as it does in Northern countries that get heavily snow inversely summer in australia is generally associated with drought and severe weather changes isn't much good for farming. (not too mention its bloody hot, too bloody hot to do a lot of work.
given how Farming people's in the north hemisphere see winter as negative due to frozen ground and failed crops is why the end of the winter solstice is revered. One could logically assume the Summer solstice would in fact be revered in aus as it denotes a change in conditions where by the content becomes more liveable as the days grow shorter, placing the New year period around december 22nd.
this is of is not necessarily the case as crops are planted and harvested all year round in different temperate zones and depending on crops so the idea that it matches up with harvesting season like the North hemisphere becomes…….washy.
then ends up completely irrelevant because most crops were introduced by europeans and there was no farming to create New years around crops. this is reflected in the fact that native peoples in australia didn't really celebrate new years at all and instead seasons in most places revolved around what was available to eat in what areas at what time's, for example the Kakadu is considered today to only have two seasons (wet and Dry) but tribes in the area use to denote as many as 8 seasons in the same area some taking many months others less then a month.