In a nightmare start, Whanganui got the territory but eagerness to shift the ball led to two steals with wingers Kalavini Leatigaga and hat-trick scorer Sireli Buliruarua making them pay instantaneously by dashing away for long-range tries.
On the first, a loose pass from halfback Lindsay Horrocks from the ruck missed two receivers and was snatched up by South Canterbury flanker and captain Nick Strachan to fire straight to Buliruarua - who sprinted off for 7-0 in the fifth minute against the run of play.
Then, a flat pass on the blindside by centre Kameli Kuruyabaki was gobbled up by Leatigaga, who had a 15m head start on everybody, and while Whanganui winger Alekesio Vakarorogo motored across from the far side to make a great cover tackle, his fellow No 11 was able to bounce over and score.
In a game that was an intense staring contest, Whanganui had another "blink" moment when, on their try-line, a simple missed Vakarorogo tackle was all Buliruarua needed to stagger through and dive in for 21-0 in 17 minutes.
As well as the pinched balls for turnaround tries hurting, Whanganui tried to overcome the wind through veteran first five Craig Clare attempting cut-out balls, cross and chip kicks, with South Canterbury seemingly inviting space a little further wide. It ultimately proved a trap.
Whanganui might have been able to withstand a 21-0 deficit at halftime, knowing they would then get the big wind behind their kickers, but four minutes from the break, No 8 Semi Vodosese couldn't hang onto a midfield cut-out pass, and South Canterbury immediately fired it out to Buliruarua, with Leatigaga at his side.
The wingers uncorked a brilliant kick-chase try, with Buliruarua getting the recovered ball back from Leatigaga to dive straight over near the posts.
Heartland's leading points scorer, Sam Briggs, slotted his fourth straight conversion, and other than those four key moments, all the home side had to do was defend hard, look for turnovers, then harness the wind to put Whanganui back in their own territory.
Coming into the second half and the sinners all made amends – Horrocks producing a great steal at the back of a 5m South Canterbury scrum to feed flanker Jamie Hughes to dive over in the 47th minute.
Kuruyabaki made some powerful runs, Whanganui getting the better of time in South Canterbury's half as second five Ethan Robinson produced strong kicks, while reserve midfielder Timoci Seruwalu came on to start busting tackles.
With Briggs down injured in the attacking line, Robinson put fullback Dane Whale through the gap. He set up Vakarorogo, running in at the corner to score, with Robinson slotting a good conversion for 28-14 in the 63rd minute.
South Canterbury, again, smartly did everything they could to slow play while defending grimly in their half, using the clock as their 16th man.
Entering the last 10 minutes, Whanganui needed to score immediately, at least for a bonus point before having a shot at the equalising seven-pointer and received a succession of infringement penalties.
But South Canterbury tackled hard and forced two big mistakes, which were followed by a scrum penalty, and after their forwards drove nearly 30m into Whanganui's half from the lineout win, the visitors had run out of territory and time.
Disappointed Whanganui coach Jason Caskey knew his side had left themselves too much to do.
"Two intercepts, made us chase our tail a lot - 60m at the other end makes the day tough.
"The biggest problem, there was space there, but we just couldn't get to it. They defended the inside."
Twice bitten, thrice shy, after the stolen tries Caskey said Whanganui were "shell-shocked" and started going with constant one-off runners, which suited the Cantabrians' big pack just fine.
Briggs and veteran halfback Willie Wright, followed by his deputy Theo Davidson, took care of the rest.
"Knowing the ground, they took some real big yardage off us," said Caskey.
"We had to make the most of every point we needed, but fell short a couple of times on the line."
While other results in round 6 of the competition going their way meant Whanganui still hold third place, they are now 10 points behind South Canterbury and eight behind future opponents Thames Valley, who just have to negotiate a home derby game against winless King Country this coming weekend.
The teams that finish third and fourth will contest the Lochore Cup final - the secondary Heartland trophy Whanganui held once in 2014.
"We've just got to box on and see what happens," said Caskey.
On the injury front, lock Josh Lane hurt his shoulder and neck from a fall at the lineout.
South Canterbury 28 (Sireli Buliruarua 3, Kalavini Leatigaga tries; Sam Briggs 4 con) bt Whanganui 14 (Jamie Hughes, Alekesio Vakarorogo tries; Ethan Robinson 2 con). HT: 28-0.
