The Abuse in Care Inquiry believes there may be more than 100 unknown people who spent time at the Lake Alice Psychiatric Hospital's child and adolescent unit.
It was revealed last year the number of people abused in care since 1950 was up to 253,000, with the number of people passing through care being six times more than previously thought.
The Lake Alice child and adolescent unit operated from 1972 to 1978. It is known abuses – including sexual, physical and psychological - happened at the Manawatu-based unit and the Inquiry is concerned there are people it does not know about.
An Inquiry investigation into the unit indicates a far higher number of patients were admitted than previously thought. At redress hearings last year, the Health Ministry told the Inquiry's Commissioners it had not been able to make a complete list of patients.
Investigation counsel assist lead Andrew Molloy said they know of around 200 people.
"To date, our investigation suggests that this number is likely to be about 50 per cent higher."
Molloy said this stemmed from a raft of "new and valuable information" obtained through interviews with former patients and staff.
Frances Joychild QC, who is representing some of the Lake Alice Survivor Group, said "it's a grave concern" and "absolutely appalling".
"We know that the records were very poorly kept."
Joychild said they already knew a lot of electroconvulsive therapies had not been logged.
"What was going on?," she asked. "All these children were in the care of the State."
She said undocumented survivors would be in their late 50s and 60s now.
"Some of them would have died, but … we've had a few family members come forward and we've been able to look at their parents' files."
Joychild said it was "in the interests of the country … the family, and the whanau that we do have the proper information, once and for all, about what went [on] out there".
"How did it happen?" Joychild said.
"There are very serious issues that have to be answered."
Joychild wants answers from the Government about its record keeping, among other things, and wants the Inquiry to "make determinations" on other issues at hearings about Lake Alice in June.
The Inquiry investigation team wants to hear from former patients, staff and their families, ahead of the hearings.
Information can be shared by calling 0800 222 727 or at www.abuseincare.org.nz.