There is a facebook page called Avon River Park calling for support from those interested in the recreational possibilities for the land soon to be abandoned (thanks for the tip Fiona). The facebook page is here.
Not wishing to rain on someone else's parade, it is unlikely the abandoned areas will become recreational land. The government has clearly said it is keeping ownership with a view to the land being remediated and redeveloped some time in the future. The payout package for the land is a large cost to the government, and they will not want to pass up an opportunity to get their money back. The nature of the redevelopment is unknown, but rumour has it that interest has already been shown from overseas.
Of course some other political party may feel driven to promise to protect the land from foreign ownership, and maybe even keep some or all of it as reserve land to compensate for that taken for new subdivisions. Maybe the new subdivisions could be levied for reserve purposes to compensate for the lost green space. CERA could cooperate with the council on this.
If you do have strong feelings about this speak with your local MP, or the political party you support. Whatever you do to support this idea please remember that the land is still home to many families, a large number of whom are grieving about having to be removed.