New Zealand were left sweating over the fitness of Kalyn Ponga after he failed to finish an electrifying debut on Thursday in front of 41,255 at Allianz Stadium with a suspected broken arm.
With Ponga off the field late, Bostock leapt high to score the winning try in the 75th minute by climbing over Dolphins' teammate Jamayne Isaako for his third of the night.
Bostock's brilliance ensured Australia narrowly avoided their first home defeat in 11 years, with the Kangaroos all but certain to finish top of group A ahead of games with Fiji and the Cook Islands over the next fortnight.
There were question marks over many of Kevin Walters' selections, but Bostock, who mainly plays as a centre for Redcliffe, vindicated his coach's faith in his ability to play on the wing.
"I don't think I've seen a better debut game from a winger or from any Australian player," Walters said of the 23-year-old's display.
"He was just in everything good about the attack, the defence, his athleticism. He should be really proud of his performance."
Other Walters decisions paid off, including the call to give Brisbane superstar Reece Walsh first shot at the fullback jersey.
Walsh showed few signs of rust in his first game in two months, and there will be significant intrigue through Australia's campaign as to whether Walters settles on the Brisbane No.1 or James Tedesco as his first-choice option.
But Walters is likely to face a headache in the halves after Mitchell Moses limped off soon after halftime with a calf injury.
"He's not too good, we'll get some scans for him," Walters said.
The shortcomings of the Kangaroos' forward selections were also highlighted against a monstrous Kiwi pack.
In front of the biggest crowd for a Sydney Test since the 1970s, the Kangaroos were manhandled and dominated by the Kiwis, who ran for close to 300 more metres.
But New Zealand lacked polish and it will be their inability to ice such games that will invite more questions about their credentials as genuine challengers to Australia's World Cup stranglehold.
A big part of New Zealand's chances will hinge on 28-year-old fullback Ponga, who scored twice in the second half on his Kiwis debut, the best of which was when he sidestepped Walsh and left him for dead.
However, Ponga failed to finish the game, with the Newcastle captain picking up a suspected wrist injury.
"It's a possible fracture of the arm but he's going to go for scans and we're not going to make any call on him right now," said Kiwis coach Stacey Jones.
"There's lots of stuff to be proud of and lots of stuff to like but there's a lot more to go in this tournament."
Even before Ponga trudged off, the Kiwis had the opportunity to put Australia to the sword but failed.
Casey McLean scored the opener after three minutes, before Tom Trbojevic and Bostock scored intercept tries for the home side.
With Kotoni Staggs sin-binned for a professional foul and Moses limping off after halftime with the scores at 10-10, Ponga put New Zealand back in front when he pinballed his way through the Australian line.
And despite Ponga weaving past Walsh for a breathtaking try, the Brisbane man had the last laugh.
Between Bostock's second and his match-winner, Walsh weaved around Ponga with a brilliant solo effort.
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