Everything north of the construction isn't part of Maybury Reserve, in fact it's all fenced off if I remember correctly.

(as an aside, it is crown land, owned by the Ministry of Education... maybe these ruins are from a former school?)

either way I don't think it should be mapped as part of the park - the actual extent of the park is visible on https://geomapspublic.aucklandcouncil.govt.nz