it most definately is a sand grabber or break away sinker used in fishing. The prongs dig the sinker into the sand in strong currents and stop the rig from drifting about. When pulling it back in the prongs flip round to enable retrieval

heres a modern version http://www.ballistictackle.co.nz/products/copy-of-cr001-long-cast-rig-with-bos-sinker

"It was actually a marsupial, closer related to kangaroos and wallabies than any of those other animals. It was originally found in Australia and New Zealand,"

Sorry - marsupials have never been native to New Zealand.

We do have possums that have been introduced though.

As an interesting side note - we only have 1 native mammal. Its a bat, but it gave up flying to walk around on its "hands" (well, more like its knuckles) and feet in the underbrush

for those interested in this kind of stuff, go here - http://www.christchurchquakemap.co.nz/

timelapse of the big one and the continuing after shocks (30+ a day) using google maps and geonet data.