Goals from Kevin Nolan, Joey Barton and Shola Ameobi ensured the Magpies did what they had to do at London Road, but a mathematical miracle could still rob them of a place in the Premier League.
If Newcastle were to lose their five remaining games, and Forest were to win all five of theirs, a 26-goal swing could yet see Chris Hughton's side stranded in the Championship.
That is all but impossible of course, and Newcastle will seal their own fate anyway provided they claim at least a point from Monday's home game with Sheffield United.
With his side facing two games in three days, Hughton made five changes to the side that beat Nottingham Forest on Monday, with Barton coming in for his first start since last September.
The changes appeared to affect the home side early on, and Newcastle's slow start was punished when Peterborough took the lead in the 11th minute.
It was a shocker from the visitors' point of view, with goalkeeper Steve Harper kicking team-mate Fabricio Coloccini instead of the ball as he attempted to clear from outside his area.
The ball looped tamely to Dominic Green, and the Posh midfielder calmly lobbed into an empty net from 25 yards.
Things almost got worse when Sergio Torres fired narrowly wide shortly after, and Newcastle's entire first-half performance was both error-strewn and lackadaisical.
They managed to create two chances midway through the first half, but both fell to Nolan, and Peterborough goalkeeper Joe Lewis was equal to each of them, saving the midfielder's ten-yard strike before parrying his close-range header.
Lee Frecklington wasted another chance to extend the hosts' lead four minutes before the break, shooting wide under pressure from Jose Enrique, and Harper was forced to tip over Charlie Lee's header to keep Peterborough's advantage down to one goal.
Incredibly, it wasn't even that at the break, as Nolan scored his 14th goal of the season to hand Newcastle a parity they did not deserve.
Ameobi played in the overlapping Enrique on the left, and Nolan bundled home the Spaniard's low cross from inside the six-yard box.
One goal at that stage was fortunate but, incredibly, Newcastle doubled their tally three minutes after the break and, inevitably, the man hogging the headlines was Barton.
Gabriel Zakuani fouled Ameobi on the edge of the area, and Barton curled a precise right-footed strike into the bottom left-hand corner of the net.
Leon Best wasted a glorious opportunity to extend the Magpies' lead shortly before the hour mark, but after being played through by Barton, the Irishman snatched at his shot and fired wide of the post.
However, it did not take long for the visitors to add a third. Best rolled the ball into Ameobi's path, and while Joe Lewis got a hand to the striker's 22-yard strike, he was unable to prevent the ball entering the net via the inside of the post.
At that stage, the away supporters were desperately hoping for news of a Bristol City winner at Ashton Gate, but they had reasons to be nervous with 14 minutes left as Peterborough pulled a goal back.
Nolan inadvertently flicked on Exodus Geohaghon's long throw, and Liam Dickinson bundled home his shot despite a decent attempt at a save from Harper.
PETERBOROUGH (4-5-1): Lewis; Gilbert, Zakuani, Geohaghon, Griffiths; Rowe, Frecklington, Green (Simpson 46), Lee (Bennett 56), Torres (Mackail-Smith 46); Dickinson.
Subs (not used): McKeown (gk), Day, Mills, Reid.
NEWCASTLE (4-4-2): Harper; Simpson, Williamson, Coloccini, Enrique; Pancrate (Routledge 46), Smith, Nolan, Barton (Gutierrez 87); Best, Ameobi (Butt 83).
Subs (not used): Krul (gk), Kadar, R Taylor, Lovenkrands.