The forecast loss has grown $9.3 million from an original $30 million, tournament organiser Martin Snedden told the Weekend Herald.
The new estimate was directly related to downgraded expectations of ticket sales.
Ticket sales are the only way the Government and New Zealand Rugby Union can make money to offset the losses from hosting the tournament.
The Government has agreed to pay two-thirds of the losses and the NZ Rugby Union will pay the rest.
Mr Snedden said it was expected 1.5 million tickets would be sold out of a total 1.7 million. This was based on New Zealanders buying 1 million.
Meanwhile local television networks negotiating a joint bid for Rugby World Cup broadcasting rights yesterday reported "significant progress" and said ministers would be updated on Monday.
The Maori Television Service (MTS), TVNZ and TV3 are working on the joint bid.
MTS will lead the bid under a deal brokered by Prime Minister John Key and Maori Affairs Minister Pita Sharples which averted a bidding war between MTS and TVNZ -- both backed by taxpayer funds.
The aim of the joint bid is to deliver nationwide free-to-air broadcasts of the 2011 tournament.
MTS chief executive Jim Mather has said there was no guarantee a joint bid would be accepted by the IRB.
