"It's a nice, folky and mellow start. High country jazz," he said about his performance, during which he was joined by two of his children.
That fitted in with the soaring peaks and tussock surrounding the festival site on Ohau Downs Station.
Grenell - then known as John hore - started his career in music when he won a Joe Brown talent quest in Dunedin in 1962.
By the end of the 1960s, he had sold more than 120,000 albums.
Since then, he has become one of New Zealand's best known and most prolific recording artists with more than 20 albums and numerous hit singles.
But recently, life has been rocky. He had almost a year off because of ill-health, which included having his mouth reconstructed and a hernia operation. Last year, he lost his first guitar to a court bailiff, who visited his Windstar Farm near Coalgate in inland Canterbury and confiscated property for an unpaid $900 traffic fine.
Grenell said he bought the Gallatone guitar when he was 11 years old from a cousin at Naseby. The Justice Department auctioned the guitar with his other property, but the person who bought it returned it to Grenell.
While he does not play it, that guitar went with him last year on a New Zealand-wide "Highway of Legends" tour featuring Grenell and other singers from the Joe Brown stable.
Now 64 years old, Grenell plans to continue performing, but he is unsure how much he is allowed to earn after July when he qualifies for national superannuation at age 65.
Yesterday, he teamed up with son Oakley and daughter Aniria, both established performers, for the opening act.
Grenell had sage advice for festival patrons: "Swing easy. Don't overdo it - there is a long way to go."
