A third major exhibition at Wellington's $180 million convention centre has failed to break even.

The City Council is refusing to quantify the deficit for the Doctor Who event, which ran from last June to October.

It also won't release specific visitor numbers, citing commercial sensitivity.

Councillor Diane Calvert says ratepayers have a right to know the exhibition's actual cost against the budget.

"Because if we don't deal with this openly, how can we look for improvements and how can we action those improvements that I'm sure we can do?"

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