Pictures captured by pilot Trevor Wright show water filling dry salt plains that lie 700km (435 miles) north of Adelaide.
The area around the lake has been inundated with rain over the last week, with falls of over 150mm reported.
Kati Thanda-Lake Eyre fills only a few times in a century. When this happens it is Australia's largest lake.
Subsequent rainfall both around the lake and in its vast catchment will determine how large the lake becomes this year. Read more on bbc.