Cooper Cronk also touched down as Australia underlined their dominance of the code and gained revenge for their 2008 final defeat when New Zealand fought back from 16-12 down to win 34-20 and claim their first world title.
Australia led 16-2 at the break but any hopes of another Kiwi fightback were extinguished immediately after the restart when Slater scampered away for his second try.
Their victory was built on the back of quick work around the ruck and precision kicking which allowed the Kangaroos to dominate territory and possession.
A ferocious defensive effort meant New Zealand barely had a sniff of crossing the line at Old Trafford, and by holding them tryless it meant Australia did not concede a four-pointer in the tournament since their opening victory over England.
"I am extremely proud," Australia coach Tim Sheens told the BBC.
"We came over in front of a great crowd and performed really well. Our defence was great and our attitude was terrific.
"We didn't start the game thinking that (hold New Zealand tryless) but with 20 minutes to go we did and to their credit they really dug in."
After an early penalty to Johnathan Thurston New Zealand suffered a blow when Roger Tuivasa-Sheck, who scored twice in the semi-final and eight times in the tournament, was forced to leave the field with a recurrence of the shin injury that put him in doubt for the match.
Shaun Johnson, who scored the last-gasp winning try in the semi-final win over England, replied with a penalty of his own but that was the last time the Kiwis got close as their bitter-rivals upped the tempo.
After forcing New Zealand to drop out from their own goaline, a perfectly placed chip by Thurston allowed Slater to outleap Kieran Foran and dive over from close range.
Melbourne Storm halfback Cronk got the second score, latching on to the ball after a break and grubber by Darius Boyd, after which Thurston overtook Mick Cronin as international league's highest point scorer with the resulting conversion.
While New Zealand would have been hoping of another second-half fight-back, any lingering hopes were dashed immediately after the restart when interplay play down the left by Smith, Thurston and Boyd freed Slater for another simple try.
On 52 minutes Morris ensured the New Zealand reign as world champions would come to a disappointing end, reaching the ball first after a kick-ahead by Jarryd Hayne.
He collided with an advertising hoarding after touching down, reminiscent of the incident that saw Australia forward Luke Lewis miss most of the tournament after dislocating a shoulder in a pool match against Fiji.
With the match effectively sealed the Kangaroos did not let their defensive effort slip, comfortably repelling the few opportunities New Zealand created and with eight minutes remaining Morris scored his second after a Hayne intercept.
New Zealand coach Stephen Kearney said their rivals were simply too good.
"I thought Australia's performance today was outstanding, they were ruthless today," he said.
"For us to have an opportunity we needed a lot of things to go our way but none of it happened for us. They saved their very best for this performance and we couldn't match them today."
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