The forward netted seven minutes into second half stoppage-time as the visitors twice came from behind to pick up a Wessex League Premier Division point in front of a bumper crowd of 502 at the OnSite Group Stadium.
Gary Austin’s rebound effort had cancelled out Kieron Roberts’ opener in the first half.
It appeared as though substitute Zak Sharp’s poked strike had won it just past the hour mark, prior to Sterne’s smart turn and finish late on.
Fareham boss Pete Stiles felt his side were at least deserving of a point for their derby showing.
He said: ‘A point is no more than what we deserved.
‘We played well, played good football, we were a bit dead on our legs at the end but we thoroughly deserved at least a point.
‘It’s great for Kai, that’s two goals in two games, he’s doing alright for us.’
It was the Royals who went ahead inside seven minutes. Lee Wort fed strike partner Roberts who raced away and fired into the far corner for his fourth goal in two games following his FA Cup treble against Horndean.
After the early setback, Fareham began to assert themselves on the derby clash.
Josh Benfield’s dipping strike hit the crossbar on 34 minutes, with Austin quickest to react to put away the rebound.
The Reds remained the more dominant but it was Portchester who regained the lead when Sharp poked home after keeper Dan Kempson could only parry Sam Pearce’s powerful header on 62 minutes.
Roberts crashed an effort against the post and Marley Ridge saw his chipped attempt clawed away by Kempson as Portchester looked to wrap up victory.
However, they failed to extend their lead and Sterne’s superb turn and finish ensured both teams still remain without a league win this season.
But Royals boss Dave Carter questioned where referee Nicholas Whittington found so many minutes of second half stoppage-time from.
Carter, who handed a Royals debut to his one-time Moneyfields keeper Steve Mowthorpe, said: ‘Where the referee found seven minutes from (stoppage-time) I don’t know.
‘It’s frustrating. We’re still gelling as a side.
‘We should have taken our chances first and second half.
‘Marley (Ridge) went through and he’s got to put the game to bed and make it 3-1.’
AFC Portchester: Mowthorpe; Colson, Da Costa, Pearce, Bailey, Ridge, Dinsmore, Ramsey, Wort, Roberts, Barker. Subs: Sharp (Da Costa, 45), Fennemore (Ramsey, 76), Searle, Briggs, Pitman.
Fareham: Kempson, Jones, Willcox, Moody, Tattersall, Cooper, Mackney, Robbins, Woods, J.Benfield, Austin. Subs: Slade (Austin, 64), Sterne (Woods, 64) Bennett (Tattersall, 75), C.Benfield, Kesseck.