One of the international ratings agencies has downgraded Chorus' credit rating to the lowest level considered to be investment grade.
Moody's has downgraded Chorus to BAA3 from BAA2 with a negative outlook, and about $US1.5 billion of Chorus debt is affected.
Standard & Poor's still rates the telephone lines company at BBB, the equivalent of Moody's previous rating.
Moody's said its downgrade was largely prompted by the Commerce Commission's decision late last year that, from December this year, Chorus must halve its wholesale price for using its copper wires to deliver broadband.
But the ratings agency said higher-than-expected capital spending and operating costs were also to blame.
Chorus spokesman Ian Bonnar said it was disappointing Moody's had not waited to see the outcome of its efforts to mitigate the impact of the Commerce Commission's decision.
The company was looking at all options available to improve its position, including cutting discretionary activity, repricing commercial services, generally managing for cash and assessing capital management options.
"We've also started constructive discussions with Crown Fibre Holdings and while this work continues, we cannot finalise our medium-term strategy or our capital management settings," Mr Bonnar said.
The company had had no indication that Standard & Poor's intended to change its rating, he said.