It was meant to run along the 141st meridian line of longitude.

But longitude is measured with time references to Greenwich Mean Time — and watches were not that great in the 1800s.

The result was a 3.6-kilometre gap where the borders were meant to have met.

"Fundamentally the difference came down to a discrepancy that the nation had at the time," Mr Burdett said.

"The longitude that had been determined for Fort Macquarie [Australia's reference point] was out by two minutes.

"Hence ours was out by two minutes and the discrepancy ended up in that border."