The Telecommunications Industry Ombudsman (TIO) has recorded a sharp increase in the number of complaints about the National Broadband Network (NBN).

The Ombudsman's annual report for 2014 to 2015 shows the total number of complaints has fallen for the fourth year in a row, dropping 12.4 per cent last year to stand at more than 124,000.

But the number of complaints about the NBN increased by 70 per cent to 6,700 — mostly because of delays in connection and missed installation appointments.

Acting Ombudsman Diane Carmody said the number was not a surprise, as more people are signing up for the NBN, but the delays are extremely frustrating for customers.

NEW COMPLAINTS PER 1,000 CUSTOMERS NATIONAL RATE 5.8 VIC 5.9 NSW 5.1 SA 5.1 QLD 4.4 TAS 4 WA 3.9 NT 3.1

"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 said.

She said the TIO had met with the telcos and the NBN about the issue, but it is up to them to improve their communication.

"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.

The report shows there was a 21 per cent drop in the number of complaints about mobiles.

Ms Carmody said that was because investment in infrastructure has improved coverage, and fewer complaints about excess data charges.

There was a slight increase in complaints about landlines and internet connection.