The Natalia Republic flag was adopted on December 24 1839. The Natalia Republic was a Boer Republic established close to a year after the Battle at Ncome River. The Republic was later annexed by Britain in 1843 & most of the Boers there then trekked into the Transvaal & Transorangia where other Boers were establishing themselves. The first few minutes of the following video mentions this flag including a number of other 19th cent Boer Republican flags which are located in the Cenotaph Hall under the Heroes Hall at the Voortrekker Monument.
The above picture shows the Natalia Driekleur / Tricolour at the right.