Taxpayers will hand over €47m a year for the next five years to pay for the iconic National Convention Centre in Dublin's docklands, which was last week revealed to be without proper sewage facilities. It is due to have its gala opening in early September.

Under the multi-million euro deal worked out between the government and Spencer Dock Convention Company Ltd (SDCCL), the state will pay €47m a year for the next five years and €23.9m a year for the following 20 years to the company for building and running the controversial centre.

This works out at a total outlay of €713m, making it one of the most expensive state projects, on a par with the Luas and the Port Tunnel. The first monthly instalment of just under €4m is due next month and these will continue until 2015 after which the payments will drop down to just under €2m a month until 2035. The centre will then revert to state ownership.

The Spencer Dock company is primarily owned by Treasury Holdings, run by developers Johnny Ronan and Richard Barrett, whose loans have been taken over by Nama. Ronan and Barrett are on the board of SDCCL, which is chaired by Dermod Dwyer.

The board also includes former head of the Department of the Taoiseach, Paddy Teahon, who resigned from the controversial Campus Stadium Ireland project some years ago.

Responding to criticism of such a massive spend on a conference centre in such straitened times, a spokesman for the OPW said the total payment of €713m over 25 years is equivalent to €350m at today's prices.

The deal with the Spencer Dock company provides for "penalties for non-performance and profit-sharing in circumstances where profits exceed specified levels", said the OPW spokesman.

While the spokesman was unable to say what the precise share-out of the income from the centre will be, he said the long-term benefit to the state will be the boost to the country's business tourism industry.

A spokeswoman for the Convention Centre said that to date it has 83 confirmed bookings up to 2013 including the International Bar Association, which will bring 4,000 delegates to the centre.

Based on a recent study by the University of Limerick which indicated that a typical international delegate will spend €1,400 in hotels, bars and restaurants during a stay in Ireland, the confirmed bookings to date will result in an €82m boost to the business tourism industry, the spokeswoman added.

But by 2013 the taxpayers will have paid close to €150m to the centre, leaving a net loss of almost €70m.

Fine Gael TD Olivia Mitchell pointed out that the centre will have to attract revenue of €4m a month for the next five years if this "huge government investment" is to pay off.

"In fact, it must and should do much more than wash its own face," added Mitchell.