Peter Byers, who was the first commercial surfboard manufacturer in New Zealand, died in 2011, aged 71.
His widow, Margaret, said she and Bruce Byers decided to sell to the council after extension of the Hillary Trail was mooted, but public ownership was always on the cards.
"The Blow Hole and Tennis Courts are a great drawcard, they are remarkable, and it's nice it is going to the public for access."
Mrs Byers said it was also the end to years of worry about trespassers starting bush fires and fishers drowning from the steep rocks.
The block had been on the council's acquisition plan since the 1970s, because it stood between two parts of the park, said councillor Christine Fletcher, who chairs the parks, recreation and sport committee.
"It is one of the most important natural areas in New Zealand and is close to a big population base.
"It has ecological and recreational significance and is an excellent investment in the future for Auckland's tourism."
Money for the land came from the council's park acquisition fund, set up by the former Auckland Regional Council.
Mrs Fletcher said that money was set aside for some properties which were critically important for the regional parks network.
Waitakere Ranges Local Board chairwoman Sandra Coney said the purchase would open up spectacular views of wild and untamed coast and add some great new walks.
"The drama of this coastline has long been recognised by artists such as Eric Lee Johnson and Pat Hanly and now the public will be able to see its rugged beauty for themselves."
The popular Mercer Bay Loop Walk, south of the new property, could now be linked to the Tasman Lookout Track to the north to form a safer alternative route for the Hillary Trail by avoiding a crossing of the busy main road.
The 70km Hillary Trail is named after Sir Edmund Hillary who holidayed in the area and prepared for his expeditions walking the steep cliff tracks. His son Peter Hillary said the Hillary family was delighted about the council's purchase.
"We love the coastline and there are parts walkers have not been able to access.
"There's a growing interest in walking the trails network - a lot of young people are out there."
The area also has a number of ecological classifications which recognise its landforms, vegetation and wildlife.
It has traces of Maori habitation, including a headland pa, midden and terraces, and remnants of one of a few cottages in the bay.
For many years it was farmed by the Ussher family, who settled Piha in the 1860s. In the 1950s, Ivy and Budge Phillipson ran The Blow Hole Tearooms.
Bruce Byers, a former marine engineer, is an earthmoving contractor in West Auckland.
His brother Peter caught the surfing bug in 1958 and became known in New Zealand and Australia for custom-made surfboards.
