Lake Alice was a maximum-security psychiatric facility on the outskirts of Marton and operated from 1950 until 1999.
Hardcastle died at the facility in 1960 and was buried in an unmarked grave at Mt View Cemetery.
Rankin used old council records and maps to locate Hardcastle's grave.
Locating Hardcastle's grave sparked a wider investigation into unmarked graves, with Rankin convinced there would be more.
He said it was difficult to get information, as privacy laws stopped him from accessing any hospital records.
"There's quite a bit involved in finding them. A lot of the information has been lost and other parts are sketchy."
He looked through a list of all the patients who had died at Lake Alice and began working through it, looking for returned servicemen and their graves. Information for people who went through Lake Alice was often hard to come by.
Through his investigations, he discovered the final resting place of 31 servicemen.
"I didn't think it was fair this had happened to people, they were just put into the ground and forgotten about.
"Back in the '60s and early '70s, when most of these people's deaths occurred, there was a bit of a different outlook on shell-shock. Many of the families didn't want to know anything about it. A lot of them weren't advised if someone did die.
"It's terrible. It was a really bad situation. When a lot of the funerals took place it would be the undertaker, one of our members and possibly a minister and that was it."
Marti Eller, Deputy Head of Veterans' Affairs, said Veterans' Affairs was very pleased to be able to support Rankin and acknowledged the hard work and care he had put in to locate the last resting places of veterans in Mt View Cemetery.
"It's important for the families of those veterans and for us. This work honours their service to our country."
Veterans' Affairs had helped with determining service records of the deceased veterans and may potentially fund gravestones and plaques.
The Marton RSA would like to hear from anyone who may have had a relative or knows of a serviceman who was at Lake Alice Hospital, as they would like to invite those people to the commemoration and blessing.
Contact Barry Rankin at barryrankin1@gmail.com, 06 327 5111, or 027 725 5234.
The Marton RSA, in association with the Rangitikei District Council and subject to Covid-19 restrictions, is now planning to hold a commemoration and blessing of the graves at Mt View Cemetery, State Highway 1 (500m north of the Calico Line intersection), on Saturday 5 March.