Tew said: "Japan are on notice that they've got to perform at a higher level as time goes on.
"Their win against the Bulls over the weekend was very timely."
Sky TV had been very supportive of the changes, which mean the competition will have fewer games. But he said there should be higher quality contests, and more in New Zealand's time zone.
New Zealand fans believed there were too many mismatches, and did not like the way the finals series was constructed - two major reasons why the 18-team format has been dumped.
South Africa, once a giant of world rugby, is struggling but it is not inconceivable that they will push for a return to five teams down the track. South Africa had demanded a sixth team but TV and attendance figures there and in Australia had been concerning from the outset of the 18-team competition.
New Zealand's playing strength and audience has clearly given them the upper hand in negotiations.
Tew said: "We must be careful not to come across as arrogant. We brought in some private equity which has also brought energy and new ideas.
"We think we bring real strength to the competition. If the maths works out, we could have all five teams in the finals."
"We were concerned the 18 team competition wasn't working - our fans gave us some pretty good feedback.
"Ideally if I was representing the views of our players, we wouldn't be going to home and away derbies but you talk to the guys running the clubs and they're not unhappy.
"I was talking to a couple of the Highlanders boys after the game (against the Blues) and they'd prefer it wasn't the case and we'll be taking that forward in the next development of the competition post 2020.
"It is necessary right now to stay in the conference model. It works from a commercial point of view for us.
"We're two years into a five year deal...we faced the reality of South Africa demanding a sixth team in Port Elizabeth and we lived with the 18 team comp. To their credit they've now listened.
"They've got their own financial pressures and as you can see from the number of players they are losing overseas they've got some big issues.
"I just commend the leadership of South Africa. They've taken note of reality. Don't forget the human element, the fans, players with families, a whole lot of stuff going on.
"There will be pain - and its not unusual for us to get the blame."