"There are exceptional deals and there is also no doubt they [European and Japanese clubs] are targeting a younger group and spending more money on that younger group," Tew said.
Because of that, it was "not getting any easier" to retain players.
The drive to contract critical All Blacks for the long term also reflects the financial confidence of the NZR, who have more money to play with as a result of an improved broadcast deal that will take effect next year.
Brodie Retallick and Julian Savea are thought to have benefited from the improved financial landscape - both are understood to be on contracts worth about $800,000 a season.
These contracts are close to the highest NZR have offered, as they felt securing those two was essential - just as it was vital in early 2011 that Richie McCaw and Daniel Carter were tied into four-year deals.
The challenge is trying to forecast now who is going to be a must-have All Black in 2019.
It is not only about assessing how good certain players could become, it is also about looking at the overall picture to determine in what positions the All Blacks could be weak or under-resourced down the track.
Cane and Barrett were teammates of Retallick in the New Zealand under-20 team who won the 2011 Junior World Championship, and are players the All Blacks believe will have a major role to play in 2019.
Cane, who has just turned 23, has been the country's best openside this season, playing encouragingly like McCaw did at the same stage of his career.
As for Barrett, his star shines ever brighter as the full extent of his talents is realised. First-five may not necessarily be his natural home, but the All Blacks know he has to be on the field somewhere, such is his ability to create and exploit space.
Cane is off contract this year and is expected to announce before the World Cup that he is staying in New Zealand.
It is believed he may be persuaded to sign a four-year deal - one the NZR were prepared to offer as not only is Cane the heir apparent to McCaw's No 7 jersey, but he is also seen as a potential captain.
Barrett has signed until the end of next year, but his likely impact at the World Cup and the inevitable overseas interest that will develop mean the NZR will be keen to engage him in extension talks early.
Tew said the union would like to sign more players on longer-term contracts, but "there are only a few players who make that length of commitment. There will still be a lot of players who sign one- or two-year deals."
Aaron Smith, TJ Perenara, Patrick Tuipulotu and Dane Coles are also off contract next year and their ages and likely importance to the All Blacks in 2019 mean all will be offered deals to keep them in New Zealand for at least another three years.
Sonny Bill Williams is another who is contracted only until the end of next year and, as he turns 30 in August, the NZR may be happy for him to continue with his preference for short-term deals.
By next year, it is probable that the NZR will be looking to persuade emerging players such as Damian McKenzie, Akira Ioane, Lima Sopoaga and Brad Webber to stick around until 2019.
Kieran Read, Sam Whitelock, Ben Smith, Aaron Cruden, Malakai Fekitoa, Owen Franks, Charlie Faumuina, Israel Dagg and Cory Jane have all signed until 2017.
2019 All Blacks (possible side)
1: Joe Moody
All Blacks scrum guru Mike Cron has high hopes for Joe Moody - the 26-year-old ex-wrestler who has incredible core strength.
2: Dane Coles
The 28-year-old took giant steps last year and is relatively young for a hooker. Should be in his prime by 2019.
3: Charlie Faumuina
Another relative baby for the position he plays. Faumuina, 28, could be the best tighthead in the world in four years.
4: Brodie Retallick
World Player of the Year at 23 - that's a ridiculous achievement for a lock. Brilliant now; could be just scratching at the surface of his potential.
5: Sam Whitelock
Has 50 caps but is only 26. Martin Johnson wasn't anywhere near his best until he was 30.
6: Akira Ioane
At 19, has incredible pace and power and is already showing an appetite for the nasty stuff.
7: Sam Cane
It's unthinkable, but Richie McCaw might not be missed - as a player - in the next four years.
8: Kieran Read
Turns 30 later this year but if McCaw can captain the All Blacks at a World Cup when he's 34 ... why not Read?
9: Aaron Smith
Might not be able to go much beyond 2019, but should still be able to play at breathtaking pace in four years.
10: Damian McKenzie
Has adapted easily to Super Rugby and brings a dynamism and sense of adventure to the role. Might be a flip of the coin between him and Cruden.
11: Julian Savea
The bus will still be charging along - leaving behind a trail of destruction.
12: Sonny Bill Williams
Keeps himself in immaculate condition - if his desire remains, his body will hold up.
13: Malakai Fekitoa
He's got everything he needs physically - his development is all about learning his craft and the nuances of the role.
14: Ben Smith
Too good a footballer to imagine he won't be able to adapt and stay relevant.
15: Beauden Barrett
Give him a free reign to do whatever he feels like - and pop up at first receiver.
