New Zealand rugby's playing stocks have always been the target of overseas clubs since rugby turned professional in 1995, but principally for players heading towards the end of their careers.
NZR, however, have lost several players in the last 12 months, such as Aaron Cruden, Charlie Faumuina and Lima Sopoaga, all of whom were considered still in their prime and Tew said there had been increased interest in younger players.
"We have been concerned about our player stocks for some time, particularly the French market, but there's a bit going on in England and Japan as well," Tew said in comments published by the Daily Telegraph in Britain.
"There's been more heat than there has ever been... clubs are increasingly talking to agents about players at a much younger age.
"It is teenagers, sometimes 15-year-olds, being approached about signing by clubs in both union and league. That's scary. We have to be very careful."
Tew thought the change of World Rugby's eligibility rules in 2020, which would require a player to reside in a country for five years before being able to be selected for the national team, was having an effect now.
Currently players who have not played for their country of birth can switch to another if they have been resident for three years.
"With the five-year eligibility rule coming in that may or may not help," Tew added. "We may see them target younger players even earlier and try to qualify them for the five years.
"What we are seeing at the moment is younger players being offered longer contracts.
"Unfortunately, it is getting younger and younger to a point we are going to have sit down to make sure we are not talking to young men before they are ready to make those type of decisions."
Domestically, NZR have long been competing for talent with rugby league -- particularly in its largest city Auckland -- and were involved in a recent tug-of-war with rugby league's New Zealand Warriors over fullback Etene Nanai-Seturo.
The 18-year-old was selected for the All Blacks Sevens team for the Sydney leg of the World Sevens Series earlier this month despite having signed a five-year contract with the Warriors.
Nanai-Seturo signed the contract as a 15-year-old.
NZR reached a confidential agreement with the Warriors earlier this month for his release.
(Reporting by Greg Stutchbury; Editing by John O'Brien)
