"Yes it does [exist]," Joyce said, drawing laughs from the crowd.
"I'm very very concerned about it. I genuinely am concerned about the level of debt that a Labour Government ..."
Robertson interjected, stating he had been forced to address "two weeks of you making things up".
"Steven you have actually embarrassed yourself again this morning," he said. "We have put out a fiscal plan that has been fully costed, independently audited and not one economist in the last two weeks has backed what you have just said right now.
"For me this is Trump-like, it's fake news and it is an attempt to distract from all of the issues and the problems that we've been discussing today. You're actually making a fool out of yourself."
Joyce countered that Labour's own numbers showed the country's debt after five years would be $11.5b higher than how the economy was currently tracking.
"To believe it wouldn't be any higher than that you'd have to believe firstly that a Labour Government would spend less than a National Government, you'd have to believe that Labour would deliver two $0 budgets ... and you'd have to believe that all these proposed taxes would have no impact on the New Zealand economy," Joyce said.
"If you believe all of that then I have a very good railway company to sell you."