The Zimbabwean frontman scored two before half-time to put Pompey in a commanding position after they had totally dominated Reading for the first 45 minutes.
Completely against the run of play though, Irish midfielder Stephen Hunt headed a goal seconds before half-time to give the under siege Royals a fighting chance.
That soon transformed into an equaliser as David James came rushing out of his goal three minutes into the second period only to make a horrendous lack of judgement to see Dave Kitson steal the ball and then produce a finish of real quality from the touchline.
At 2-2 the game then merely increased in velocity as both sides neglected any defensive duties and a goal bonanza ensued.
Hermann Hreidarsson thrust his side back into the lead by heading in Sylvain Distin's cleverly chipped centre, before Benjani sealed his hat-trick by chasing Sulley Muntari's throughball to round Marcus Hahnemann and slot into an unguarded net.
Croatia midfielder Niko Kranjcar than capped an inspiring display to get in on the act and as he arrived late at the far post to head home from close range from Sean Davis' cleverly chipped centre.
Amid Pompey's goal frenzy, Reading had the misfortune of missing a second-half penalty as Nicky Shorey's spot-kick was saved by James following Distin's handball inside the area.
The away side though were not to be outdone in the scoring stakes as substitute James Harper's deflected drive cannoned off Shane Long and into the net.
True to the pattern of the game, just as Reading had looked to have clawed their way back into the game once more Pompey grabbed a sixth as Davis' long-ranger clipped Michael Duberry leaving Hahnemann helpless.
Shorey atoned for his penalty miss by getting on the scoresheet ten minutes from time, but it was merely academic as Muntari wrapped things up at the death with a penalty.