Corporate customers on Spark's broadband network are experiencing issues in the lower South Island.
The problem is affecting customers in Queenstown and Invercargill, but it may be more widespread.
"We are looking into it to see what the scope of the problem might be," a spokeswoman said.
Spark is working with Chorus to resolve the problem.
It has been established Chorus equipment is likely to blame for the fault, she said.
"There are teams of people trying to get it resolved," she said.
It is still unclear how widespread the issue is, how many customers are affected and whether personal customers are among those affected.
Attempts to contact Chorus have so far been unsuccessful.
Dunedin infrastructure company Delta and the Southern District Health Board were among those affected.
A Southern District Health Board spokeswoman said the fault had prevented staff from accessing the internet, but the health board's critical and patient systems were still fully functional.
