Hey guys i found this in my back paddock. Well actually my granpa found it ages ago, and its been in my shed for ages. I didnt know i had it untill my sister pointed it out to me. Unfortunately dear grandpa is deceased, so i dont know were the site was.  
There arent any tell tale signs of Maori occupation, no pa sites (earth wrok castles) or villages nearby.
Most likely this was an artefact dropped in transit.
So what is it, i think it could be a hammer or something?

Photo0005 by greencheapsk8, on Flickr
 

Photo0003 by greencheapsk8, on Flickr
 

Photo0004 by greencheapsk8, on Flickr

To me it looks like a pestle (as in mortar and pestle), a device for grinding grain or similar.

Quote from curious_aardvark on May 31^st, 2011, 6:49am:

 
No chance of an axe head, its about as sharp as a big doorknob
Im going with a punding stone.  
Might take it to the local museum, see what the curator guy says

Is there taro root in that area (Hawiians mash taro root with coconut milk and call it poi) i dont know if Maori eat taro but there may be other fruits or vegetables they prepare that way also

Me thinks it's a pestle.I R pretty sure the shape,size,and type of stone is just ideal for that purpose.

It's surely is a pounder,but how did the curator know that it's a flax pounder and not a pestle?A heavy,hard wood fibre pounder is much easier to make and at least as effective,but that stone tool would be much better for grinding,than one carved from wood.They surely wouldn't carve something from stone if wood works as good or better.

Oh yeah, i gorgot about you overseas guys
New Zealand flax is like a thin flexable, more fiborous agave.

 There is one problem with the pestle theory... there is no wear!!  If it was a pestle that had seen use it would be much smoother all around the large end from grinding against the mortar.
  It may have have been a fishing weight, net weight, anchor etc. as it looks to me exactly like stone plummets I have seen found in North America.  
   I don't know a lot about your area, but if my memory is correct there were also stone clubs that looked kind of like that, though I think they were commonly made of greenstone.  
 
  Recently I found these, a pecked and ground granite axe and a pecked granite maul.  They both have a groove all the way around meaning they are the oldest type.  They were made very much like your artifact, by taking a harder tennis ball sized of flint or quartz and hitting it against the granite, each time you hit it a piece the size of a b.b. comes out, that's why they call it pecking.  The axes blade was finished by grinding on a wet slab of sandstone.  
  Yours doesn't show any signs of being ground or of wear at all, just pecking.  It is also possible that it is unfinished, but darned if it doesn't look like a very large fishing plummet.
 

 
 
 
 
  Here are 3 photos of fishing plummets I pulled off of Google Images: