It's been a great week or two for Auckland's volcanoes. First came their listing as one of eight proposed world heritage sites. This coincided with a letter to Auckland civic leaders from Minister of Conservation Chris Carter saying he was "keen to see public agencies working together to ensure the protection of these important landmarks which give Auckland City and its neighbours so much in terms of urban character".

Then this week, the plans of state highway builders Transit New Zealand to build a motorway over the Hopua volcanic tuff ring at Onehunga got the red light and Manukau City announced it was spending $6.07 million of reserve funds to buy the Pukaki Lagoon explosion crater in Mangere to create a new premier park.

The 61ha Manukau harbour Pukaki site, 2km from the airport, went on the market in March, advertised as suitable for six lifestyle blocks. Previously, the city council had tried to protect the site with a heritage zoning but the owners successfully appealed against that in the Environment Court in 2001. To the city's credit, when the crater site went to tender, it moved quickly to save one of the best preserved and least developed explosion craters left in the Auckland volcanic field.

A Victoria University drilling programme in the crater in 2000 suggested the volcano was between 80,000 and 100,000 years old, and that its 26m layer of sediment contained the most complete record recovered of ancient climate and volcanic eruptions in the Auckland and Taupo-Rotorua fields.

More recent drilling by Auckland University staff indicate it could in fact be twice as old as their Victoria colleagues thought.

Until 1911, the 36ha interior flat land was a tidal lagoon. But Parliament handed control to the Auckland Harbour Board.

Despite protests from the local Pukaki Maori, the board leased the land to a local businessman and car importer, George Henning, who dammed the basin and turned it into a speedway circuit, attracting 9000 spectators to the first meeting in 1929.

With the dissolution of the harbour board in 1989, the "lagoon" ended up in Manukau City hands, which then handed the freehold title to the Pukaki Maori Marae Committee. The new reserve will be jointly managed by the city and co-owner, the local tribal authority.

Now all we need to make this purchase even better is to somehow sneak Pukaki's twin, the neighbouring Selfs Crater, into the proposed park.

This purchase is just the sort of positive action needed to demonstrate to the Unesco World Heritage Committee - and to the Government - that we Aucklanders do take our stewardship of this unique volcanic field seriously.