As at June 2009, reserves were 183.1mt, but resources were...

As at June 2009,

reserves were 183.1mt, but resources were 1815.2 mt.

Using resources, at future forecast production of 10mt, that's 181 years worth. At current 5mt, that's 360 years worth.

Resources aren't as accurate as reserves, because they haven't dug it up yet obviously, but it's reasonable to assume they've got plenty.

Source:
http://www.macarthurcoal.com.au/Operations/ReserveResourceStatement/tabid/122/Default.aspx