Midfielder Keith Andrews put the Tractor Boys ahead in the 23rd minute of Saturday evening's Championship clash but Darren Ferguson's men hit back in stunning fashion to lead 4-1 at the break.
Paul Taylor - making his first start for Posh - drew them level in the 30th minute, Tomlin scored eight minutes later and both men found the net again in quick succession to put the hosts in total control.
Paul Jewell's side were dealt another hammer blow on the stroke of half-time when midfielder Lee Martin was shown a straight red card for a foul on Mark Little.
Ipswich defender Tommy Smith was then sent off for a professional foul shortly after the restart and Grant McCann scored twice, the first a penalty, before Tomlin completed his treble late on.
The result condemned Ipswich to a third defeat from their opening four games and left them looking anything but the serious promotion contenders they were tipped to be.
After a lively opening in which chances came and went, Ipswich led in the 23rd minute when Andrews collected possession and hit a stunning volley into the top corner.
But the lead last just seven minutes as Taylor connected with Little's cross and hit a superb 25-yard strike into the top corner.
Ipswich struggled thereafter and found themselves 2-1 down in the 38th minute when Tomlin showed delightful skill before drilling home his first from 12 yards.
Remarkably, the hosts struck again two minutes later when Taylor swept a right-footed shot into the net and it soon got worse for Ipswich.
Peterborough went 4-1 up in the 42nd minute when McCann found Tomlin with a fine pass and the attacking midfielder broke clear to drill a right-foot shot into the net.
The visitors were on the ropes and their frustrations boiled over on the stroke of half-time when Martin caught Little with a late tackle and was promptly shown a red card.
Shortly after the restart, Ipswich substitute Smith fouled Tomlin on the edge of the 18-yard box as he threatened to burst clear.
Referee Graham Scott showed Smith a red card and pointed to the penalty spot, with McCann stepping up to fire home left-footed with a fine spot kick which gave David Stockdale no chance.
Posh claimed number six in the 56th minute when McCann hit a low shot from 12 yards which deflected into the net.
Ipswich knew the game was up but it could have got worse for them in the 57th minute when George Boyd hit a fine shot which drew a brilliant save from Stockdale in the Ipswich goal.
Stockdale denied Boyd again in the 64th minute and 10 minutes later Posh came close to a seventh when substitute Ryan Tunnicliffe fired against the crossbar from six yards.
There was more misery for Ipswich late on, though, when Tomlin completed his treble with a neat right-footed finish from close range.