"I don't think it's made a huge difference to date," Tew said of Williams' inclusion on ticket sales.
"But we're seven days out now and our guys are working incredibly hard to provide a programme of activities that focuses on the Sevens."
He said NZ Rugby hoped for crowds of 12,000-15,000 each day of the two-day tournament.
"But that's going to depend on people's decisions next week and the weather and a whole lot of other factors, I guess."
Tew said there had never been a better priced tournament and that NZ Rugby had packaged the event in a variety of ways to encourage fans to attend.
He hoped as the teams arrived in the capital this week - two or three were already there - and as New Zealand arrived, the hype around some of the players would increase interest.
"I think that'll help and we'll start seeing some ticket sales pick up a little bit more than we are now, because they are still quiet but we'll keep working at it."
Wellington has hosted the event since 2000, and tickets were sought after. They sold out in four minutes for the 2011 event.
But in recent years crowd numbers have dwindled.
And in the past two years the tournament has faced ongoing pressure from the NRL Auckland Nines, which attracted a sell-out crowd of approximately 45,000 people a day for the inaugural two-day tournament in 2014, with numbers dipping to just above 40,000 last year.
"Those days [of quickly selling tickets] are clearly gone," Tew said.
"But the trouble with an event like ours is that once you're not selling it out, there's no compelling reason for people to buy in advance. So they can wait."
Wellington's hold on the sevens has been up for debate for the past five years.
And Tew said late last year that Wellington faced a " little bit of a challenge" if it wanted to retain the event in the face of ever-decreasing ticket sales.
Officials from Dunedin's roofed Forsyth Barr Stadium have previously expressed an interest in nabbing the event.
Tew was confident the quality of the tournament would be just as high, however.
"We will run a very good tournament - operationally it will be top notch. Great for the players, and that's the most important thing."