I understand that's one reason for the successful defense of Rorke's Drift: those Zulus had outrun their supply line and had to pull back.
It is true that the 4,000 Zulus at Rorke’s Drift were a phalanx of the 20,000 Zulu army that defeated and killed 1,300 British troops at the Battle of Isandlwana in January, but I’ve never read anywhere that they retreated from lack of supplies.
I might add that the primary Zulu warrior’s weapon was the assegai iron spear, and while they did possess a few rifles, they weren’t technically trained to use them. Thus, I don’t know what kind of supplies they might have outrun. As Nepalese natives, I’m sure they could have lived off the land, as they had done for centuries.