"We will lose money on the Rugby World Cup," English, who is also finance minister, said during a trip to meet finance chiefs in Hong Kong and Singapore.
"These events... in my view their direct economic impact is overestimated. You can always find a consultant who will tell you that an international event or a new stadium will make you much richer - that is not true," he said.
English did not specify the extent of the losses he expected, but earlier this month Rugby World Cup chief Martin Snedden denied claims that the cost of hosting the tournament had blown out to NZ$500 million ($400 million).
Snedden described as "disingenuous and misleading" a New Zealand Herald report that said staging the largest event ever held in the country would cost NZ$1.2 million but generate just NZ$700 million in direct economic returns.
He backed predictions from the Reserve Bank of New Zealand governor that the September 9-October 23 tournament would provide a NZ$700 million boost to the economy, lifting growth by 0.33 percentage points.
English said on Thursday the event would "be a big boost for confidence... and will put us on the world stage" as well as countering misconceptions that New Zealand was "shut down" after twin earthquakes that hit Christchurch this year.
"The biggest economic impact will be the fact that major international sponsors are bringing so many of their clients to New Zealand who would otherwise never come," English said.
"That gives us an excellent opportunity to showcase New Zealand."