Mr Docherty was from Marblehead, about 30km north of Boston, where his mother Trish lived. He also spent time in Santa Barbara, California, where his father lived.
Mrs Farrell said Chanel, with whom Corey was very close, had been in New Zealand about a month and her brother arrived in the country roughly 10 weeks ago.
Mrs Farrell said the location of Corey's memorial service was yet to be decided.
She said her grandson had already made friends in New Zealand despite his short time here.
Mrs Farrell did not wish to talk in more detail but she earlier told the Marblehead Reporter newspaper her grandson saved up enough money to fly to New Zealand and live here for four months.
"He didn't want to come out of college and have two weeks' vacation," Mrs Farrell told the paper. "He wanted an adventure.
"He went to various places in the county, made new friends his age and had a wonderful time, so it was satisfying he did live that dream," she added.
Meanwhile, friends of Mr Docherty have already raised more than $17,400 for his family on Giveforward, a fundraising website.
A friend said Mr Docherty, known as Dickey Doc, was "one of the best, most lovable and adventurous guys that one could know."
After hearing Mr Docherty had passed away, some people made gestures to remember their friend by writing "RIP Corey" messages on landmarks in his hometown.
Mr Docherty attended Marblehead High School, then graduated from Roger Williams University last year.
