Hi Judith,
Many years ago I paid the NSW registry for a 20-year search for a death, after failing to find it in the UK, other Australian states, or NZ, and was told "not found". This was my third attempt to find it in NSW records. When later years of the deaths index were finally put online I found it in minutes, with one letter in the name which differed from what I had written. A very obvious alternative, and I had asked them to check variations of the name.
So, now that there are agents who are permitted access to the volumes, it might be worth getting one of these to double-check for you, if this is the reason that you think she lied about her place of birth at her marriage (that you have already had a search done for her). An agent could check through the years around the age she gave looking for her forename and possibly slightly changed surname in all NSW districts. Other than that suggestion, it is more likely that she came from another Australian state or New Zealand, than the IOM.
I'm assuming that her marriage and death certs were no help? Were the parents given on her death cert the wrong ones? Did they differ from the ones on her marriage cert?
Sue