Auckland Zoo had eight chimpanzees in 1990, which lived in two groups:
The tea party chimps (Bobby, Janie and Josie) lived in one group in the old cages; while the hand raised females (Suzie and Sally) lived in the grassed exhibit with a young mother raised pair from Taronga Zoo (Mike and Lucy) and their infant son (Lucifer), who was born at Auckland Zoo in 1988.
Mike was imported in 1982 and integrated with Suzie and Sally in the hopes of breeding from them. They were behavioural non breeders and so Lucy was imported in 1985, which resulted in the births of Lucifer in 1988 and Luka in 1993. Suzie and Sally never bred.
I’m unsure why you only saw three chimps in the other group, but it appears they were indeed Mike, Suzie and Sally (since you didn’t see an infant); and so Lucy and her infant son must have been in the dens. They were 100% an integrated troop of five at that point. There’s some nice photos of Mike, Lucy and Lucifer from 1988 in the zoo’s history book and I found footage online a few years ago of the troop of five together in 1989 (the link is now invalid).