He was taken to hospital less than a week after being knighted in a ceremony at his home by Governor-General Sir Jerry Mateparae.
The investiture, on Wednesday last week, was brought forward at his family's request because of Sir Paul's failing health.
Sir Paul had been treated for prostate cancer earlier in his career but it returned early last year, putting his work on hold. Last June he also underwent open-heart surgery.
Sir Paul briefly returned to the country's screens and airwaves, but in December announced he was retiring from broadcasting.
He has spoken of his grim health problems.
"We still have a lot of fun, a lot of good times, but soon realities have to be faced ... We haven't had a chance to meet with the doctor to discuss what's going on, but I don't think it's flash.
"I don't think Houdini will do it this time."
Sir Paul has survived a car crash, fatal helicopter accident, light aircraft crash and previous illnesses. He had been looking forward to retiring in his Hawkes Bay home.