The home side took the lead after 16 minutes when Gabadinho Mhango's powerful free-kick was only palmed back into danger by goalkeeper Darren Johnson, only for Moses Mthembu to score from the rebound.
Usuthu came close to equalising midway through the first half when Taariq Fielies picked up out Athini Maqokolo but he could not apply the finish.
Richards Bay then suffered a huge injury blow when the influential Mhango was forced off with what looked like a serious injury, and was seen in crutches at the end of the game.
Amazulu again came close to drawing level early in the second half when Maqokolo went one-on-one with Richard Otieno but the home shot-stopper came out on top.
The hosts sealed maximum points from the contest six minutes from full-time when Ngema struck a sweet volley from the edge of the box that beat Johnson after the visitors failed to clear their lines.
It was a first win in six matches for the Natal Rich Boys and moves them up to ninth in the league standings.
Amazulu, meanwhile, missed the opportunity to go third in the table after suffering their fourth defeat of the campaign.
Orlando Pirates brought the curtain down on their 2025 campaign in style after dethroning the Carling All-Stars with a deserved 2-0 victory in the Carling Cup final at Moses Mabhida Stadium on Saturday.
Former Bafana Bafana striker Shaun Bartlett believes South Africa may finally have found a player capable of threatening the national team's long-standing goal-scoring records.
Mamelodi Sundowns stars Iqraam Rayners and Bathusi Aubaas have earned a major career boost after signing a lucrative endorsement deal with global sportswear giant Puma.
Former Kaizer Chiefs midfielder Mandla Masango believes the club is showing clear signs of progress this season, even as questions continue over their co-coaching structure and inconsistency in front of goal.
