The two teams were locked at 13-13 at Rodney Parade with just a few minutes remaining but Geraint O'Drsicoll's superb break from well inside his own half was the decisive moment.
And when he fed the flying Frewen, there was only ever going to be one outcome.
Earlier, Dominic Franchi and Leon Andrews had exchanged tries with O'Driscoll and Dorian Jones booting the rest of the points.
But Frewen's score proved decisive as he put Keys' play-off hopes in doubt.
Keys had taken the lead just seconds into the first half, Jones landing a monster penalty from the half way line but it didn't take long for derby passions to stir.
Visiting skipper Rob Nash and Newport lock Dafydd Rosser were involved in an off the ball incident which bizarrely resulted in Nash not only losing his shirt but also resembling Tarzan with his long hair billowing behind him.
Wayne Davies reprieved the pair but it was Newport who were dominating territory. Ross Coombs, brother to Welsh international Andrew, looked to have forced his way over but referee Davies decided the ball had been held up.
And after Jones had cleared the danger, the fly half added a second penalty to give his side a 6-0 lead, somewhat against the run of play.
The visitors had seen little of the ball, save for one Nathan Trowbridge break but had the chances to pull further clear. Jones failed to make it three from three when pushing another long range penalty effort wide and Newport responded with the first try of the game.
O'Driscoll was the creator and when replacement Owen Broad chipped ahead, it was a foot race to the line. Keys had a number of men covering but none were a match for the pace of Franchi who reached the ball first.
O'Driscoll converted and saw a penalty effort strike a post but at half time, the Black and Ambers led by a single point.
Only one word could sum up the second period - scrappy. Jones put an early penalty effort wide and with the boot dominating the game, errors were the order of the day. Newport were seeing most of the ball with Andy Tuilagi the obvious threat. The huge centre was at the heart of the action but even he failed to break down what was a stubborn Keys defence.
Balls continued to be spilled but it was Keys who regained the lead when centre Andrews found just enough room to stretch over despite the attentions of a number of tacklers. Jones added the extra two.
In a game of few clear chances, opportunities had to be taken when they came. But Newport's Tom Hancock failed to make the most of the men outside him, throwing a pass into touch when a score looked certain.
It mattered little though and two further O'Driscoll penalties, either side of a miss from Jones and a yellow card for Keys' Ryan James, saw the scores level going into the final 10 minutes.
But a mediocre second half burst into life with Frewen's late score as Newport took home the bragging rights.