He's not eligible, so would need an exemption to play for NZ as he doesn't meet art. 7 (he's played competitive matches for SA u17s at the 2015 CAF Championships so it might be more difficult to get an exemption), but before he left to SA he said he feels like more of a kiwi than a South African...
EDIT: FIFA allowed Tommy Smith to switch to NZ and he was in the same situation so it's highly possible they'd also allow Liam to switch too.