The former Orlando Pirates striker, who replaced Lunga Zondi after the break, netted in the 80th minute to cancel out Lerato Manzini's 67th minute opener at Durban's Moses Mabhida Stadium.
The result leaves AmaZulu seventh on the Premiership table with 13 points from 10 games - seven points adrift of leaders Mamelodi Sundowns, and three points ahead of 12th-placed Celtic.
The first chance fell for the away side in the fifth minute when Rodrick Mutuma found Musa Nyatama with space outside the box, but Aubrey Mathibe dived to his right to push the ball wide of the post.
Ten minutes later Nyatama had another opportunity when the ball fell kindly for him on the right side of the box, but this time he blasted his shot wide of the near post.
Ayanda Dlamini had two chances in quick succession for the hosts in the 26th minute but his first shot was blocked while his second attempt was tame and went straight to Patrick Tignyemb.
Five minutes later Goodman Dlamini saw his volleyed shot pushed onto the post by Tignyemb, while A. Dlamini’s follow-up shot was also blocked by the Celtic goalkeeper.
Ernst Middendorp’s side could have taken the lead into the break but Mutuma sent his shot wide in the 39th minute, while Manzini was denied by a good save from Mathibe on the stroke of half time.
Manzini did eventually get it right in the 68th minute when he arrived late on the blind side to bundle the ball into the back of the net.
Usuthu came close at the other end four minutes later but Nkosinathi Mthiyane shot wide of the target before A. Dlamini drew a decent low save from Tignyemb.
Mthiyane almost caught the Celtic goalkeeper off guard in the 74th minute when his corner curled onto the crossbar; the ball fell for Tamsanqa Teyise at the far post but his rushed his header wide.
The equaliser arrived five minutes later when Ndulula fired home from the rebound after his initial header had been blocked on the line by Tignyemb.
Usuthu could have taken maximum points at the death but Mohammed-Awal Issah and G. Dlamini both missed the target before Ndulula drew another save from Tignyemb in added time.