Ajose scored five goals in five appearances after his return to the club from Swindon, including a hat-trick at home to Notts County.
Posh manager Darren Ferguson told the Football League's official website: "Nicky has been magnificent since he returned from his loan spell at Swindon.
"He was unfortunate that the strikers at the football club during his time away were in excellent form.
"Now he is back and has his chance, he has taken it. I said when he returned I would give him a run of games and he has been excellent.
"I can't speak highly enough of him. His attitude has been first class and he has been fantastic in the building and, of course, on the pitch."