The Telecommunications Industry Ombudsman (TIO) says it has seen a sharp increase in complaints about the National Broadband Network (NBN).

The Ombudsman has put out its annual report for 2014 to 2015, which shows the total number of telecoms industry complaints has fallen, but complaints about the NBN specifically are way up.

The overall number of complaints is down for the fourth year in a row, dropping 12.4 per cent to just over 124,000.

There was also a 21 per cent drop in the number of complaints about mobiles, as well as a slight increase in complaints about landlines and internet connection.

However, the number of complaints about the NBN shot up by 70 per cent (to 6,700), and mostly covered delays in connecting customers and missed installation appointments.

Acting Ombudsman Diane Carmody said it was not surprising, given that the NBN has gained thousands more customers in the last year, but the delays are extremely frustrating and raise several questions.

“Why are things not going right? Why is it that when technicians are booked, they're not turning up? Why have they been perhaps diverted to some other job?” Ms Carmody asked.

She said the TIO would continue meeting with telcos and NBN about the issues, but improving services and communication is up to them.

“I think getting them together around the table is the first important thing, to work out what are those systems and to talk with each other,” Ms Carmody said.