Hodson said Welch was feeling better in the weeks that followed, but collapsed one morning on his way to the shower.
He died of a heart attack despite attempts to resuscitate him.
"Whether the Tonj Tummy contributed to it, we don't really know. He was pretty run down and exhausted with the heat."
Hodson said police showed up on his eldest son's doorstep back in New Zealand to tell him the news.
"It was just a shock for all of us. The kids are doing very well considering, and they're all being very supportive of each other."
Welch also had a partner in Papamoa who had planned to meet him in Thailand after his time in South Sudan.
His cremation and repatriation had been an expensive process due to lack of facilities in Tonj, and had cost around $15,000.
Hodson said her brother's body had to be flown out of Tonj to Juba, South Sudan's capital, where his body was embalmed before being flown to Nairobi, Kenya, to be cremated.
It was a lengthy process, because at first the coffin was 5cm too big for the plane to Kenya.
His eldest son flew to collect the ashes and returned home on Sunday to Tauranga.
Hodson said she was overwhelmed by the generosity of Kiwis who had donated to the Givealittle page set up by friends of the family for Welch's children to help cover the cost of bringing their father home.
She said it was a difficult and sad time for everyone involved, but Welch would have wanted the work in South Sudan to continue, and would have encouraged others to volunteer.
A baby born three days after his death in the ward Welch helped build was named Richard as a tribute to him.
The Ministry of Foreign Affairs confirmed it was assisting in the return of Welch's body to New Zealand. His eldest son is believed to be in Nairobi as part of the repatriation process.
In tributes posted on social media, Welch was remembered as a man always wanting to help those in need.
Friend Heather Gibbs said: "I miss you already Richard, your huge heart was always there to greet me and everyone on Sundays, I will miss your great conversations, sense of humour and the love and care you have shown me over the years. You are a top man and will be sorely missed by many. I'm so so sad."
Others also expressed deep sadness at the loss of someone "always giving to others".
In 2015, Welch joined Bryan Archer and his daughter Destinee, with her husband, Jono Macleod, in their South Sudanese volunteer work.
The Macleods said in an online tribute: "He really fell in love with the South Sudanese people, and they fell in love with him. In the evenings he'd go for walks along the river with crowds of children following him and holding his hand, and he'd head out for rides on our quad bike to soak up the African scenery.
"Since then he has remained a special friend and has always inspired us with his heart to help others and generous spirit."
