Hi Warin!
Thank you so much for catching me on this. 
As you can probably tell I'm not too crash hot when it comes to multipolygons. 
I hope that I've fixed it now with this newer edit however let me know if this isn't the case. 
I made it a relation as map share vic classifies all the creekside zones between Mahoneys Road and Merri Creek as the same parcel - Central Creek Linear Reserve.
In regards to the land use, I'm not too sure. I don't think it is a nature reserve as it doesn't fit into any of the catergorys in the OSM wiki. "a protected area of importance for wildlife, flora, fauna or features of geological or other special interest." 
It also doesn't seem to fit into the catergory of park.
I put it into the landuse - grass catergory as i think it fits it best. Being an area of mown and managed grass not otherwise covered by a more specific tag.

I'm sorry if this mistake has caused you any headaches and I'll make sure I read up on Multipolygons before attempting again!
Please let me know if theres still any mistakes in the updated version.
Regards,
Tom