The Department of Human Services is investigating how many of the repeat calls to its help lines are legitimate, after it reported a massive jump in users receving an engaged signal when trying to contact the agency.

DHS revealed last night in a budget estimates hearing that 42 million calls had received an engaged tone when attempting to contact Centrelink from June last year to April 2017.

It represents a huge increase on the prior 12 months, when 29 million calls were blocked, and on 2014-15, when 22 million callers were told lines were busy.

The department revealed it had asked its managed telco services partner Telstra to investigate how many of the calls were actually frustrated users just trying to get through.

It said it suspected smartphone apps that allow users to automatically redial a number every few seconds played a role in the increased engaged signal figures, as well as individuals trying to perform denial-of-service (DoS) attacks on the help lines.

CIO Gary Sterrenberg said DHS had asked Telstra to track the phone lines over four weeks to work out how many of the calls were unique.

"The early evidence suggests there are robo calls, but we want to make sure we're not just looking at one point in time, [rather] track it over a full month," he told the committee.

However the department said there were "undoubtedly" what it called "cyber attack" factors involved in the 42 million missed calls, intended to flood the phone lines to make them unusable. It did not elaborate.

"As you know we don't talk a lot about it but it is an ongoing issue," acting head of Centrelink Barry Jackson said.

Telstra has been told to exclude calls that have been blocked because of factors outside of DHS' control - like issues at an exchange - as well as calls from numbers that have tried to get through under a certain number of times and within a certain timeframe.

"Obviously it's reasonable for people who don't get through the first time to try again, but it's actually not reasonable to try a thousand times a day, so we want to strip that out," Sterrenberg said.