Basic trigonometry is at the core of the method that Norwood used in the 1600s, and this method applies for the whole world, not just Australia
Bryson summarised it in A Short History of Nearly Everything
"Starting with his back against the Tower of London, Norwood spent two devoted years marching 208 miles north to York, repeatedly stretching and measuring a length of chain as he went, all the while making the most meticulous adjustments for the rise and fall of the land and the meanderings of the road. The final step was to measure the angle of the sun at York at the same time of day and on the same day of the year as he had made his first measurement in London. From this, he reasoned he could determine the length of one degree of the Earth’s meridian and thus calculate the distance around the whole. It was an almost ludicrously ambitious undertaking—a mistake of the slightest fraction of a degree would throw the whole thing out by miles—but in fact, as Norwood proudly declaimed, he was accurate to “within a scantling”—or, more precisely, to within about six hundred yards. In metric terms, his figure worked out at 110.72 kilometres per degree of arc."
The French Geodesic Mission did the same the following century, with the same result, within reasonable margin of error for the time
The method used has no meaning on a flat earth. The trig is based around circular geometry and trig, and falls apart if the Earth is flat. Subsequent revisions have merely refined the figure to a more exact one, and/or confirmed the oblateness around the equator.