The future of a much loved garden sanctuary on the shores of Sydney Harbour has been secured after the New South Wales Government agreed to hand over the land on a 30-year renewable lease.

Wendy Whiteley has spent the past two decades tending the patch of land, which is known as "Wendy's secret garden", at the back of the Lavender Bay house she shared with her late ex-husband, artist Brett Whiteley, and their daughter Arkie.

But the 74-year-old had been growing increasingly anxious about the garden's future because it is on land owned by the State Government, which had been refusing to commit to keeping it as a reserve or park.

She said she is thrilled the Government has now agreed to lease the land to North Sydney Council on a 30-year renewable lease.

"It will become a collaboration now instead of having a slightly worrying feeling that somebody could arrive with a bulldozer someday or a chainsaw or something and it could all be gone overnight," Ms Whiteley said.

"I can't quite grasp it yet. It's still a bit unreal."

Transport Minister Andrew Constance said the deal meant the garden could be enjoyed by the public.

"That then gives absolute surety that the gardens are absolutely locked away for the beauty of everybody," he said.

Prior to Ms Whiteley's transformation, the patch of land was overgrown with lantana and being used as an unofficial dump ( 702 ABC Sydney: John Donegan )

"A 30-year lease with a 30-year option. I'm pretty sure it would be a game politician in 30 years' time not to continue what is absolutely beautiful."

The garden began its life as a patch of land owned by NSW State Railways but it had been neglected for more than a century. It was overgrown with lantana and being used as an unofficial dump.

Ms Whiteley began clearing the land not long after her ex-husband's death in 1992. Brett and Arkie's ashes are also buried there.

Ms Whiteley said the garden became much more than therapy for her grief.

"I've got to the stage where... it's better than that, than just being mere therapy for grieving. It's bigger and better than that because it's creative," she said.

She said she thinks Brett and Arkie, who passed away nine years after him, would both approve of the garden.

"I have no doubt that Brett would have complained about how much money I've spent down here," she joked.

"He would have made some great drawings and Arkie would have been thrilled."

Ms Whiteley has invested millions of dollars in the project and today it is a sanctuary for local residents and visitors, filled with tall Moreton Bay fig trees, flowers and shrubs.

Local Liberal MP Jillian Skinner said she often brings her grandchildren there.

"For me this is a magical place," Ms Skinner said.

"I think this is just the most incredible transformation of what really was nothing, scrub and rough dirt, to what I think is a treasure."