The Maori Queen has been admitted to Waikato Hospital's intensive care unit after she had a heart attack yesterday morning.
Dame Te Atairangikaahu's condition is understood to be serious and last night family and tribal members gathered at her bedside for prayers.
Dame Te Ata receives dialysis treatment three times a week, and has been in hospital several times in the past year.
Tribal leaders have moved to protect the privacy of the revered leader and play down the seriousness of the situation.
A statement from her secretary, Taini Rutene, said she had been admitted to the hospital for observation and medical review.
Further tests are to be carried out today and it was expected that Dame Te Ata would then return home, Ms Rutene said. Tainui chairman Tuku Morgan said the health of the tribe's paramount leader was not open for public discussion.
"The Queen's health is sacrosanct - she is the paramount leader of our people - there will be no comment about her condition."
Tainui kaumatua Hare Puke said the Queen's health had deteriorated in the last nine months.
"It has been a shock to everybody including to her."
He said the Queen had done what she could to prepare her people for the consequences of her deteriorating health.
"She has forewarned her people of her condition. She has made it clear that like all of us we live in faith and hope."
Dame Te Ata celebrates her 75th birthday next week and the 40th anniversary of her coronation was celebrated in May.