Sluggish internet speeds are driving more complaints from Australians than those about patchy mobile services.
Internet-related complaints to the industry ombudsman have hit a five-year high, surging by more than a fifth between July 2015 and June 2016.
More than 112,000 new complaints were lodged with the Telecommunications Industry Ombudsman during the year about landline, mobile and internet services.
Complaints about mobile services fell by more than a quarter to their lowest level in nine years, but it was a different story for internet services with complaints up 22 per cent.
"Consumers told us that slow data speed was the biggest problem with internet services," Ombudsman Judi Jones said on Thursday.
"New complaints about internet data speed increased by 48 per cent."
Complaints about the NBN more than doubled during the year and made up 12 per cent of the total complaints lodged with the ombudsman.
"Delays in connections to the network, faults including unusable services, and dropout of services were regularly reported, which is of concern," Ms Jones said.
But she said the number of complaints were not growing at the same rate NBN was adding customers.
Despite the jump in internet-related complaints, most issues raised with the ombudsman still concerned mobile services, which accounted for more than a third of all complaints.
"Complaints about mobile services have reduced due to better product offerings from telcos including higher data allowances and increased investment in mobile infrastructure," said Ms Jones.
Across landlines, mobiles and internet services the main reasons for complaints were billing, customer service, faults, complaint handling and contracts.
TOP COMPLAINTS FOR TELCO CUSTOMERS:
1. Billing and payments
2. Customer service
4. Complaint handling
(Source: Telecommunications Industry Ombudsman's 2015-16 Annual Report)