3 people in one private grave isn't unusual. Perhaps you have more space to spread out in Australia! If the number exceeds 6, then it's either a posh vault for a dynasty or (more likely) a common grave, where I've seen up to 50 burials.
The problem here, though, is the plot number. Present numbers at HG are 1 or 2 letters for the section and a number for the grave. I assume /Con/ means Consecrated, i.e. Church of England rather than Roman Catholic or Nonconformist, so perhaps it's near the Anglican chapel. Either there's another column in the register that hasn't been transcribed, or the section naming was different in earlier times. I suspect the latter, having been defeated before by some strange plot specifications at HG. Reading between the lines of other posts, I think someone here has a deceasedonline sub and can look around the register pages and maybe gain some clues from other burials. Otherwise, Phil is a lot closer than me so I'll hand it over to him to look around or ask in the office.