Mayor Len Brown's latest budget forecasts a debt-to-revenue ratio of 265 per cent.
A one-notch downgrade, he said, would lead to an $11 million rise in interest costs.
Interest costs are funded by rates. A $14 million increase in running costs equates to about a 1 per cent rates rise.
To create headroom, council officers have suggested drawing down $100 million over each of the next two years from a diversified investment portfolio, currently valued at $335 million, to manage the debt ratios.
Ms Crone said the decision to draw down the fund was all very last minute, lacking in information and haphazard.
"We have no idea of how much room we have before we max out. Is it $20 million, $100 million or $200 million.
"Councillors should be given more information about how much headroom there is to make a prudent decision," she said.
Ms Crone said before liquidating $200 million in assets the council should look at its revenue and spending lines.
"Len Brown is recommending to spend more more than what is in the (long-term) budget and sell assets to pay down debt. But if you spend less you don't have to sell assets," Ms Crone said.
She was critical of the council's lack of an asset sales strategy. Ms Crone said the debt issue was "another step towards a financial crisis" in Auckland.
A review last year of council assets by two advisory firms, Cameron Partners and EY, recommend a range of asset sales.
"Why go for a rainy day fund? They are going for the easiest and quickest way," Ms Crone said.
The "rainy day" reserve fund is part of the liquidity policy of council, a budget report says.
Last December, ratings agency Standard & Poor's acknowledged Auckland Council's strong financial performance and financial management but noted a "very high debt burden", which has risen steadily from 196 per cent of operating revenues in 2012 and is expected to peak below 260 per cent.
"Downward rating pressure might arise within the next two years if Auckland's debt increases to more than 270 per cent of operating revenues as the council borrows to invest in more infrastructure without offsetting revenue growth," it said.