Family and friends of the retired police sergeant are concerned about his condition, described by his wife Danielle as "critical".
Mr Nicholson has been been receiving chemotherapy and other treatment in Dunedin Hospital since Sunday.
His sudden illness led to the couple’s decision to cancel the April 1 Contact Epic mountain bike race around Lake Hawea, so Mrs Nicholson could solely focus on her husband’s health.
Mrs Nicholson said 75% of entry fees would be refunded and no entries would be rolled over to 2024 because the future was uncertain.
"I have emails that I have been sending out each day [to entrants] and there are a lot of facts on those about where he is at," Mrs Nicholson said.
"It is AML. It is acute and it is fast growing. He wouldn’t have had long if they hadn’t picked it up when they did.
"If it all goes well, we are looking at six to nine months in hospitals around the country. He’s now [on Thursday] on his fifth day of chemo because they needed to start it straight away."
Mrs Nicholson said she and her husband were hopeful he would respond to treatment.
Their children, Jake (22) and Sienna (19), who both live in Wellington, came south to be with their father last weekend and Mrs Nicholson made a quick trip home on Wednesday to pick up items so she could stay in Dunedin long-term.
"This is the critical time. The doctor said to me you need to go home and get everything," she said.
Mrs Nicholson said 99% of Contact Epic entrants had responded compassionately to the cancellation.
The race was also cancelled in 2020 and 2022 because of Covid, with entries rolled to this year.
Some people had asked if entries could roll over again but Mrs Nicholson said that would not happen.
"We have spent more than 25% of the entry fees but we will still honour the 75% [as per terms and conditions].
"We would have loved to have rolled it over but unfortunately we had to face the fact Aaron might not be here next year and it would be difficult to do it ..."
Wanaka LandSAR volunteers, event directors, key sponsor Contact Energy and others offered to run the event at short notice, but Mrs Nicholson, while grateful, decided she wanted to avoid the potential for more "curve balls".
