Rugby star Sonny Bill Williams has returned to his old school at Mt Albert Grammar, making a loyal "M" sign with his hands.

Williams, who converted to Islam in 2008, has posted pictures on Instagram with his wife Alana and daughter Imaan with a simple tagline: "Proud husband and father, Forever grateful to the most high."

It's good to be back on Insta #lovemygirls ❤

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In one photo, he is perched by the stone gates of Mt Albert Grammar School.

Took a trip down memory lane today..

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School spokeswoman Estelle Sarney said the Blues player had not made an official return visit recently.

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"He must have just snapped that driving past. He must have jumped out of the car and done that," she said.

"That finger sign he's making is the Mags sign, that's what all the kids do. He's a proud Mags old boy."

Williams, 31, signed a three-year deal last year to stay with New Zealand Rugby until the 2019 Rugby World Cup.