The other two in the top five, South Africans AB de Villiers and Hashim Amla, are both over 30 but showing no signs of slowing down, while the other three are between 24 and 26.
Williamson w ill be eyeing the top spot, but New Zealand batting coach Craig McMillan is wary of insisting it's just a matter of time.
''We'd all like to see him get to No 1," former international batsman McMillan said today.
''I have to say it's a pretty classy group of world class batsmen. In those five, in the next five to six years there will be some jostling among those players.
''The fact he's in that group, and his name is associated with those guys, is exciting, and great for New Zealand cricket."'
Williamson, whose average of 47.19 is the highest of any New Zealand test player with a minimum of 20 innings, earned praise from Australia's coach Darren Lehmann yesterday.
''I think he's one of the best five batters in the world. He's a very, very good player, one of the best I've seen come out of New Zealand," Lehmann said.
''We didn't bowl to him how we would've liked (in Brisbane), or how we planned, so hopefully that changes in the next test."
Smith also spoke highly of his batting rival and was asked if he'd seen any weaknesses.
''I'm not sure, I'd like to know. He's a quality player, he looked good in both innings (140 and 59), his game is together, he looked really tight with really soft hands," Smith said.
New Zealand captain Brendon McCullum stayed at No 15 on the list, but Ross Taylor slipped four spots to No 17, while BJ Watling slid two to No 25.
On the flipside opener Tom Latham climbed two, to No 35, while offspinning allrounder Mark Craig jumped eight spots to No 57.
Both leading New Zealand bowlers, Trent Boult and Tim Southee, slipped one spot to No 6 and 11 respectively. Craig drifted one place to No 40 while Doug Bracewell dropped six to No 47.
South African quick Dale Steyn continues to lead the bowling list, with England's Jimmy Anderson and Pakistan spinner Yasir Shah equal second.
The top five rated batsmen in test cricket:
1: AB de Villiers (South Africa), age 31, tests 99, average 51.92, 21 hundreds
2: Steven Smith (Australia) age 26, tests 34, average 55.15, 11 hundreds
3: Joe Root (England) age 24, 35 tests, average 54.9, 8 hundreds
4: Hashim Amla (South Africa) age 32, tests 85, average 52.0, 23 hundreds
5: Kane Williamson (NZ) age 25, tests 42, average 47.19, 11 hundreds