But when the great-granddaughter of Tom Roberts attended the opening of the major exhibition of his works at the National Gallery of Australia last month, she realised it was time the work found its rightful home.

Roberts made the sketch of his only son Caleb in 1898 as a study for a larger portrait, neither of which has ever been on public display.

But on Friday, the constant stream of visitors to the blockbuster show had reason to pause and look closer at the poignant sketch, which is now framed and hanging beside an equally touching bust of Caleb, aged nine, created in London in 1907.

Lisa Roberts, herself an artist, had kept the sketch of her grandfather in her own collection for many years, and although she had stored it carefully in acid-free tissue paper and often took it out to look at, she had never felt it had a proper place.

"It's never fitted in, it's like a maverick piece," she said.

"I've always loved it because of the spontaneity of the situation. It's clearly not for exhibition."

It was when she was wandering through the exhibition's final room, filled mainly with works created towards the end of Roberts' career, that she knew where the sketch belonged.

The room has many examples of Roberts' work at a time when he no longer needed to accept commissions for portraits, and shows his private side.

"This room was different from any other gathering of Roberts' pictures I've ever seen before, and it just brings out the relationships with his family and this was part of the story," she said.

Curator Anna Gray said the gallery had been thrilled when, weeks after the show had opened, Lisa Roberts offered to donate the deeply personal work.

She said the drawing was the equivalent of a modern-day parent capturing their child on an iphone.

"I think he's expressing his love for this child and his amazement that this child exists, and loved Caleb all his life and wanted to keep it close to him, as we do," she said.

Caleb went on to become a decorated war hero at 21, having served on the Western Front, and trained as an engineer.

He eventually settled in Melbourne, became chairman of the Country Roads Board and later director of military intelligence in the Second World War, and died in 1965.

Lisa Roberts, who grew up surrounded by the works of her great-grandfather, said parting with the sketch was a delight, and that the artist would have approved of its final destination.

"I think it's clearly an excellent drawing, and I think he would have been delighted that it's not just found its place, but it's found its time," she said.

"It's found an environment where it's understood, and valued and appreciated, and particularly in this room. These are deeply personal, he did these for himself."