He said the cost was two-fold: increased upfront capital expenditure and the impact of slower development contributions - which would have come at the end of each 10-year stage - but repayments were now pushed out to between 20 and 27 years as the development was occurring all at once.
Mr Allen said the council would "wear the gross loans" and that those loans would be repaid from the development contributions.
"So it doesn't impact on rates, it is the development community that will repay that debt."
He said the risk came in because council was opening up more land and the timeframes around developing that land may take longer.
He explained that the money had been spent and the infrastructure was in place - but if development of the land did not take place as forecast it would mean the council would not have the money to repay the loans.
"The only mechanism we have got at the moment to repay that debt is development contributions," he said.
"So the interest cost is compounded, which may mean the development contributions will have to increase to cover that and that is the risk."
Mayor Lawrence Yule said while the change was slightly riskier, it meant the council had significantly reduced the development costs to the developers.
"They are happy with this approach," he said. "We have got an upturn in the economy, a significant demand for that land and we think it is appropriate, and the council has resolved until now, that it is appropriate that we take that step."