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 are gearing up for a decisive Betway Premiership showdown as they travel to the Moses Mabhida Stadium on Saturday, aiming to leap into first place with a win over an impressive Durban City FC.
As Durban City FC prepare to host Orlando Pirates on Saturday, questions are mounting over whether the KwaZulu-Natal side can upset one of the PSL's top teams.
Kaizer Chiefs fought back to earn a dramatic 1-1 draw against Zamalek SC in their CAF Confederation Cup Group D clash at the Peter Mokaba Stadium on Saturday.
Orlando Pirates climbed to the top of the Betway Premiership table after securing a solid 2-0 victory over 10-man Durban City at Moses Mabhida Stadium on Saturday afternoon.
