Arthur said concerns about competition were addressed by setting the exchange up as a separate stand-alone entity.
"Those are the sorts of questions that have been asked in other markets," he said. "We have the luxury of a model which is quite well developed and well thought through."
The media organisations would also maintain their own ad sales teams and would still sell the majority of inventory through direct sales.
There would also be opportunity for other New Zealand media organisations to use the KPEX platform over time, Arthur said.
Financial details of the agreement won't be disclosed other than that the four media groups will be equal partners in KPEX.
Media commentator and StopPress deputy editor Damien Venuto said KPEX was similar to the Pangea Alliance, which has seen The Guardian, Economist and a few other companies team up to provide premium inventory.
"It seems to be a case of local publishers teaming up to fight against the international juggernauts that are chomping away at ad revenue," Venuto said.
"The formation of KPEX will hopefully allow for more of the money spent on digital advertising to end up in the local market, rather than being fed into global exchanges."
The online ad exchange market had been largely unregulated until now, he said.
"By creating an ad exchange filled with premium, reputable inventory, it hopefully avoids some of the issues that have in the past hindered digital advertising."
KPEX is powered by global technology company Rubicon Project.
- Liam Dann