Mrs Tarr (40), who emigrated from South Africa to Dunedin with her husband last year, has undergone two cycles of hormone treatment and operations to harvest eggs so her sister could attempt to conceive a child through in vitro fertilisation.
She went into Dunedin Hospital last week for a second attempt to have healthy eggs removed.
The procedure was successful, and the four healthy eggs provided will allow her sister to have IVF treatment in the next couple of days.
"It's an absolute relief. A beautiful gift to give somebody. It's been a long road but I would recommend it 50 million times."
Mrs Tarr had received a tearful telephone call from her 36-year-old Otago-based sister, who did not want to be named, last year, saying she had early menopause and would not be able to have children.
She and her husband had been trying for about four years to conceive.
She had gone to the Otago Fertility Service and was told egg donation and IVF treatment would be an option.
Mrs Tarr, a claims manager, and her husband, who have children of their own aged 18 and 15, were considering moving to New Zealand, so her sister's plight sealed the decision.
Two weeks after she arrived in Dunedin, she met a fertility specialist and a psychologist to test if she could be an egg donor to her younger sister.
In April this year, she went into hospital and had an egg removed from her uterus under general anaesthetic, but the egg proved too small.
A second cycle of hormone treatment had "knocked her for six", as stronger hormones were used to increase the likelihood of producing a healthy egg.
However, she was delighted doctors were able to harvest four eggs for fertilisation last week.
Would she encourage women to become egg donors?
"Absolutely. You go and sit in the fertility clinic and you see all the photos, and you don't realise how many couples are desperate to have kids. It is so heartbreaking."
