Tamahere Eventide Trust chief executive Louis Fick said the resident's test came back positive on Sunday.
As soon as they were made aware of the positive test, 27 staff were told to self-isolate.
The man who had a positive result has been in self-isolation with nurses wearing protective gear since Friday. Four other residents who he dines with have also been isolated and the rest home is in lockdown.
At this stage everyone appeared to be in good health and no one was showing symptoms, Fick said.
Fick said it was pretty taxing having 25 per cent of staff off work, but they were doing their best to cope.
The rest home was now being advised by the medical officer of health and residents who may have been in contact with the infected resident would be assessed. Staff would also be tested.
"We understand your concern for loved family members at Assisi, please be assured that we are doing all that is possible to isolate the virus while continuing our normal cares for all residents. We will continue to update you with this evolving situation," the letter said.