Manukau Mayor Len Brown faces surgery this morning after his heart attack on Saturday.
Mr Brown, 51, was taken to hospital after collapsing to the floor when speaking on stage at the Pacific Music Awards when he collapsed to the floor.
A spokeswoman at Auckland Hospital told NZPA that Mr Brown was in a serious condition that was causing concern in the intensive care unit.
She said he was due to be operated on this morning.
Acting Manukau Mayor Gary Troup said doctors had said tests were showing some encouraging signs of improvement.
Mr Troup said it was unclear how long Mr Brown would be sedated or how long it would take him to recover.
"He's a very fit guy but worked hard as well. His intellect would tell him when to slow down," Mr Troup told the New Zealand Herald.
"I'm here in the caretaking role and I hope that's short term. He's done great things for Manukau, I'm happy to keep the seat warm for him."
Mr Brown's wife Shan Inglis said her husband was an extremely hard worker.
"However, Len is also an extremely intelligent and sensible man. He knew his limitations.
"This has been a huge shock to us all as Len has been in very good health and thoroughly enjoying his role as mayor."
Mr Brown's predecessor Sir Barry Curtis, who held the job for 24 years, said the job was one of the hardest in the country.
Sir Barry told Radio New Zealand he typically had almost 1000 engagements a year and working 90 hours a week was not uncommon.
Local Government New Zealand vice-president Kerry Prendergast, the Wellington Mayor, said most mayors worked 60 to 80 hours a week.
She said it was common for first-time mayors to have overly heavy workloads and advised them to keep a good work-life balance, the Radio New Zealand website reported.
