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Samoa's Head of State, his Majesty Malietoa Tanumafili II, has died in hospital aged 95.

Malietoa passed away on May 11 at the Tupua Tamasese Meaole II National Hospital at Moto'otua, Samoa's secretary of state told state-run television. He had been in hospital for about a week.

Malietoa was the world's third longest reining living monarch after Thailand's King Bhumibol Adulyadej, who has reigned since 1946 and Britain's Queen Elizabeth II, who ascended to the throne in 1952.

New Zealand Prime Minister Helen Clark said it was a day of great sadness for Samoa.

"Through his long reign as Head of State, Malietoa represented Samoa with wisdom, humour and insight," she said.

A wave of sadness and disbelief swept through the nation as the Secretary of State Vaasatia Poloma Komiti informed Samoa on SBC TV1.

"It is with deepest regret that we inform you of the passing of our Head of State Malietoa Tanumafili II," he said.

Vaasatia said the Prime Minister would be making an official address later today.

The passing of Malietoa signifies the end to the first phase of Samoan history since Independence in 1962.

Malietoa became joint head of state with Tupua Tamasese Mea'ole in 1962, however he was the sole Head of State since 1963 after the passing of Tupua.

He was born on the 4th of January 1913, and was bestowed the title Malietoa in 1939.

"Malietoa was educated at St Stephens School near Auckland. He was a great friend of New Zealand as Head of State, and was well known to successive New Zealand governments and diplomats," Miss Clark said.

"It is significant that New Zealand has a Treaty of Friendship with only one country- Samoa- and our shared unique relationship was due in no small part to Malietoa's influence as a father of modern Samoa.

"New Zealanders of Samoan descent, together with their palagi counterparts, will be thinking of Samoa, at this sad time."

Malietoa's wife, Lili Tunu passed away in 1986. They are survived by two sons and two daughters.

It is understood that a mixture of traditional Samoan and international protocols will be observed for his funeral.