Ratings for the cost of servicing Victoria’s billions of dollars of desalination debt are being reviewed.

The Fitch ratings agency is placing desal plant operator AquaSure’s “A-minus” rating for the $3.65 billion in debt on a “negative watch”.

The shift came after a fuse blew at the desalination plant when workers tried to produce water for the first time last month.

If the rating is officially cut then the cost of borrowing will increase.

“The failure’s root cause, as well as the time and cost of repair and cost allocation, are undetermined, creating uncertainty as to AquaSure’s ability to achieve its required water production by 30 June 2017 and its ultimate long-term performance,” Fitch said.

The plant was estimated to cost $3.1 billion when work started under Bracks-Brumby Labor governments, but total costs have since hit $24 billion.

Victorian water bills are set to increase by about $12 a household from July 2017.