Telecom has asked the Serious Fraud Office to investigate an alleged breach of its database that allowed rival Slingshot to gain access to sensitive customer information.

The Business Herald understands the SFO received a complaint from Telecom on January 17, which was assessed four days later and a part one investigation commenced.

It is understood Telecom's Wireline database was breached last month allowing Slingshot, through marketing company Power Marketing, to access information such as customers' names, addresses and billing details.

The information also included prices and conditions of specific customer plans, but no credit card details or the numbers customers had called.

SFO chief executive Adam Feeley would not comment on the investigation or whether a complaint had been received. Telecom would not comment on the alleged breach or if it had laid a complaint.

No charges have been laid against Power Marketing, contracted by Slingshot to cold-call rivals' customers to sign them up to Slingshot. The Herald made several unsuccessful attempts to contact Power Marketing owner Paul Ross.

CallPlus chief executive Mark Callender, who oversees Slingshot, said Slingshot was not being investigated by the SFO.

Privacy Commissioner Marie Shroff said last month she would investigate the alleged breach that exposed sensitive customer information.

A Herald on Sunday report last month said a former Power Marketing staff member estimated the database contained 2.15 million names.

The Herald understands that Telecom has lost 100,000 customers through the alleged breach, worth roughly $60 million. Documents show the SFO is looking into numerous incidents of unauthorised access to Telecom's database.