England slumped to 86 for five soon after lunch as three wickets went for two runs in 13 balls with the out-of-form Ian Bell and Paul Collingwood both dismissed for nought.
At tea, England were still far from out of trouble at 180 for five.
But an unbroken sixth-wicket stand of 94 between Pietersen and wicket-keeper Tim Ambrose (36 not out) kept New Zealand at bay in a match the tourists had to win to level the series at 1-1.
Andrew Strauss, England's century hero in their six-wicket second Test victory at Old Trafford, was still on his lunch score of 37 when he drove loosely at a wide delivery from pace bowler Kyle Mills and was well-caught at first slip by Ross Taylor.
Then 84 for three became 84 for five when Bell, hitting across the line, was clearly lbw to Iain O'Brien.
England then lost their third wicket off the last ball of an over when Collingwood was drawn into playing outside off-stump by Mills and edged to Taylor, the hosts struggling at 85 for six.
Bell, often accused of not scoring runs when England really need them, had now managed just 45 runs in the series and Collingwood 32.
All-rounder Collingwood's exit made it 23 innings without a Test hundred since his 128 against the West Indies on his Durham home ground in June last year - the last time England managed the benchmark first innings total of 400.
Ambrose, whose maiden century in the second Test against New Zealand in Hamilton in March came after England had collapsed to 136 for five, now found himself facing another rescue mission.
South Africa-born Pietersen, who began his English county career here for Nottinghamshire before moving to Hampshire three years ago, may have made his name as a flamboyant shotmaker but, recently, he has shown himself capable of batting patiently.
He did allow himself a rare moment of extravagance when sweeping New Zealand captain Daniel Vettori to complete a 104-ball fifty after 'fetching' an off-stump delivery from the left-arm spinner.
That boundary was only Pietersen's fourth four but few could argue with the expert way in which the 27-year-old paced his innings.
Earlier, New Zealand opening bowlers Chris Martin and Mills both quickly justified Vettori's decision to field first on a soft, green-tinged pitch as they made the ball move away off the seam.
Mills bowled opener Alastair Cook off an inside edge for six.
Next ball Mills produced a near unplayable delivery to Michael Vaughan which cut away and bounced steeply.
But the England captain, who struck three boundaries in four balls, fell for 16 when he was bowled attempting a drive by first change Iain O'Brien.
New Zealand gave a Test debut to Gareth Hopkins after Brendon McCullum was declared unfit to keep wicket shortly before play because of a back problem.
However, McCullum included as a batsman after struggling No 3 James Marshall was dropped in the only personnel change to New Zealand's second Test side.
England, for the first time since 1885, named an unchanged team for the fifth successive Test.