Sir Anand Satyanand has resigned from his role as chairman of the inquiry into abuse in state care, paying tribute to survivors who shared their stories - and promising it is on firm footing to finish the job he started.

The former Governor-General told the Herald in an exclusive interview he "had to make a judgment call about age, time and capacity".

He said he had turned 75 in recent weeks and made the decision after recognising the inquiry would continue through until 2023 - longer than originally envisioned.

The workload had also expanded, with the terms of reference expanding from state care institutions to include abuse in faith-based care.

In resigning, Satyanand said it was an appropriate time to go as the inquiry was about to shift from the establishment work carried out in the first 18 months to holding public hearings and carrying out the investigation it was set up to do.

"I was responsible for the shipbuilding but I won't be the captain," he said.

Satyanand's distinguished career includes lengthy service as a judge, then Ombudsman, before being named Governor-General in 2006. Before becoming Governor-General, he chaired the confidential forum for former psychiatric hospital patients.

He was appointed to the role in February 2018 when the Royal Commission into Historical Abuse in State Care and Care in Faith-Based Institutions was established.

Prime Minister Jacinda Ardern announced the appointment, saying: "The independence and integrity of the inquiry and the process it follows are critical and Sir Anand has the mana, skills and experience necessary to lead this work."

Satyanand's departure will be seen as a blow to the inquiry, particularly for survivors with whom the Inquiry had worked hard to establish a bond of trust.

He said he recognised his departure could lead to disappointment among some of the 1100 people who had registered with the royal commission for a private hearing.

However, he said the quality of the remaining commissioners and the work carried out so far and the systems established to carry the royal commission forward should give confidence in the path forward.

"The survivors remain at the centre of all this. The scale and trauma of emotional, physical and sexual abuse that has been experienced by people can only be described using words like profound."

Listening to survivors share "difficult and heart-rending" stories of abuse was a "searing experience" which would leave only a "stony-hearted person" unaffected.

Satyanand said his resignation from the inquiry was not connected to claims made after one private hearing that he had fallen asleep, or any health issues. At 75, he regularly saw a GP and cardiologist and attended a gym.

He said he "could give a rendition of everything that took place" during the session with the survivor which led to claims he had fallen asleep.

Satyanand said the projected workload of the royal commission had expanded far beyond what was anticipated.

Initially anticipated to be three days a week, three weeks a month for three years then "done and dusted" by 2020-2021, the work had filled weeks and months since it began and was not going to finish until 2023.

"It's been a hard and unremitting task."

He said he had questioned whether he had the "capacity to work full tilt" for the entire period and decided it best to step back now.

"The skills and experience of the commissioners who remain in place leave me confident I am leaving the inquiry in good hands."

The workload "doubled in time and doubled in scope" after Satyanand pushed for and succeeded in including those who were in faith-based care.

Satyanand also pushed to ensure the Treaty of Waitangi had a place in the terms of reference, and to give the royal commission discretion to investigate abuse outside its mandated 1950-1999 period.

Asked if she was disappointed to see Satyanand step down, Prime Minister Jacinda Ardern said it was likely that the Royal Commission into Abuse in state care would take some time.

"That means we might see personnel changes from time to time," she told media this afternoon.

"What's important is that we continue their work and that's absolutely our commitment."

The royal commission was preparing for its first public hearing from October 29 to November 8, while also continuing hearing from survivors in private sessions. So far, 80 had taken place and feedback had been positive.

In leaving the royal commission, Satyanand has taken on a part-time role - anticipated to be one day a week - as Chancellor of the University of Waikato.

Royal commission executive director Mervin Singham paid tribute to Satyanand's "humble and warm" leadership, saying the inquiry was well placed to move into its next phase of work.

The scale and scope of the inquiry meant it was likely - as allowed in the terms of reference - for the royal commission to seek further funding from the Government beyond the $79 million initially allocated.

"At this stage, I would say our calculations suggest the money will not be enough."

The logistical challenge of developing infrastructure and systems had started with "an empty room with almost nothing".

He said the royal commission had leaned on experiences of similar inquiries in other countries, including Australia, Canada and the United Kingdom, so as to avoid issues which had arisen elsewhere.

But it also had to wrestle with broader terms of reference, meaning it had a greater and more logistically unwieldy task to manage.

It included developing the findings in consultation with survivors, state bodies and other interested parties to ensure the final report was one in which they had been involved.

The intent of the inquiry, as at the outset, was to make sure the abuse suffered by those coming forward "never happens again".