Auckland is rivalled only by Honolulu as the weediest city in the world, and the ranges are where the harmful plants thrive the most.
Piha's bushland is of high ecological value.
It is free of livestock, deer and goats, and has nearly been cleared of possums.
Surrounding the steep, narrow Piha road - which motorists burn their brakes on - is a canopy of native kauri, rata, rimu, totara, kahikatea and puriri.
A quarter of New Zealand's native flowering plant species are found within the ranges.
The Piha community is a hardy group who react and mobilise when their wilderness is under threat.
The three-year legal tussle over a proposed beachfront cafe, now built, was a reminder of how hard some residents will fight to preserve their paradise.
Councillor Sandra Coney says Piha residents are "get-on-and-do-it folk" who "built their baches out of car cases and waste timber".
When problems arise, they are tackled by passionate community groups.
One of those groups is Piha Coastcare, which is leading the fight against climbing asparagus, a slender, scrambling plant which wraps around small trees and saplings.
It is fast-growing and a rapid coloniser which carpets the forest floor and prevents regrowth of seedlings.
Coastcare encourages residents to spray climbing asparagus (the council provides herbicide) or dig out the plant's tubers, before disposing of them safely in weed bins.
Humans are one of the worst spreaders of weeds - cuttings dumped on the roadside can eventually spread to a regional park.
The organisation's co-ordinator, Pat La Roche, says one of the main problems in weed spread at Piha is absent landlords or tenants.
"There are a lot of people at Piha who work hard to keep their weeds under control.
"But there's also a large number of rented-out properties where people are not there or less aware of the weed situation.
"And they have wandering willy running wild in the back garden.
"There's still a lot of people who don't understand why it's important to control weeds and preserve a lot of species that are disappearing in the wild."
Biosecurity officials say it is crucial to get the mess of agapanthus, climbing asparagus, cape ivy, and pampas under control before the cost of management gets too high.
The money needed to battle these and 50 other species is expected to increase as latent weeds come out of the woodwork - some weeds can remain quiet for decades before blooming.
Climate change is also expected to fuel the spread of weeds.
Half of the old Auckland Regional Council's weed control budget went into managing the intruders in Piha and Karekare, where species in private gardens had spread into the neighbouring regional parkland.
The weeds threaten endangered plant species, transform wetlands, grow faster than natives and smother them, and put native birds and insects at risk by destroying nesting and feeding areas.
Auckland Council biosecurity manager Jack Craw said weeds were the second-greatest threat to our ecosystem after rats and possums, but were harder to control.
Weeds can be spread by birds, which carry their flowers for up to 400m, by humans who dump their garden waste, or by the wind, which can disperse seeds.
The ARC, which is now integrated into the new council, launched the Strategic Weeds Initiative in the area in 2005.
The initiative made the rare step of using council money to fight weeds on private properties, in order to prevent them from jumping the fence into regional parks.
The scheme has reduced agapanthus drastically and is now setting its sights on climbing asparagus.
Piha residents say the surf, sand and bush is part of their identity.
Relief workers cut a road down to the beach during the 1930s Great Depression.
Ever since, Piha-ites have fought to maintain their bach tradition and unblemished landscape, right on the doorstep of New Zealand's largest metropolitan area.
While weeds may seem to be an ever present problem in Auckland's temperate climate, Piha's special character depends on the protection of its forest.
Ms La Roche emphasised that the battle against weeds was exhausting, but absolutely essential.
The council's funding and programmes helped, but individual action was needed if the forest's good health was to continue.
"[Eradication] is certainly do-able, with funding and community buy-in. The ARC has done a huge amount of work, but it takes both the councils and the community for it to happen.
"The seed bank is still in the soil, the birds can do their good work, and we have a great climate for growing natives.
"If you simply remove the weeds and keep them away, the bush will regrow."
