"We could scarcely believe the generosity of the gift."
Mr Ayton said at the beginning of last month, the Project Promise appeal had raised $2 million.
"We'd set ourselves this immense goal....to come up with the final million in just two months. I was cautiously optimistic we could do it but, truth to tell, there was always an element of doubt."
The only detail which has been made publicly available about the anonymous benefactor is that they are from the Bay of Islands.
The centre, which will be built next to Whangarei Hospital, will provide day-stay cancer treatment and related nursing and support services.
Currently, Northland cancer patients have to travel to Auckland or be treated in a small facility in Whangarei, which offers little privacy and has limited space for family and friends.
The new centre will have nine curtained treatment chair spaces and six consultation rooms with beds. It will also have a special child and youth area and a family meeting room.
However some Northlanders - including adults who need radiation treatment and special care - will still need to travel to Auckland, the Northland Community Foundation said.
The final cost of the centre is expected to be around $5 million. The Northland District Health Board has agreed to contribute the balance.
A spokeswoman from the Northland Community Foundation said construction of the centre was expected to begin at the end of the year.
They were hoping to have the centre open by the end of next October.
