Absent court property orders following your divorce, or an enforceable agreement she technically remains a joint owner and is entitled to her share. However, if you have been paying for the mortgage all this time, then likely you have a strong equitable claim to a greater share but will need to apply to the Supreme Court to address this, and perhaps all of it if you can show you did have an agreement for her to sign over her half.
Alternately, if there was no formal property order dividing your property after your divorce, either of you could in theory bring an application to the Family Court for an order to divide up your estate, even many years after the divorce. To bring an application for property orders so long after the dirvorce (it is not clear how long, but I'm assuming it is a very long time if you have been paying on your own for 15 years) she/you would need the leave of the court (i.e. the court's permission to apply out of time). The court will usually grant leave if there were significant assets and it would be unjust not to now hear the application.
If she or you gets leave, the court will essentially make a determination as to what is fair having regard to the contributions of both parties to the marriage, both initially and throughout. The court will also take into account special considerations, such as whether or not there are health issues affecting one party such that it will interefere with their ability to care for themselves; whether there are children of the relationship which will be cared for by one or the other parent, and the extent to which that will affect their earning capacity.
The court will also take into account what you actually did many years ago in terms of reaching an informal settlement, which it appears you arguably did.
In theory the court can base its assessment on your assets at present, rather than when you split, however, the court will be conscious that any increase in assets since that time will have been made without any direct assistace or involvement of your spouse, which will likely limit any claim she may have on such increased assets.
It certainly isn't as clean cut as her being entitled to 50%.
If you have been served with court documents it is imperative that you seek the assistance of a family lawyer given there are substantial assets involved, as a failure to deal with court proceedings, can result in a hightly prejudicial judgment against you in your absence (if you send me a copy of the supreme court papers you refer to I will try to confirm whether these indicate proceedings are on foot).
If you need help locating a lawyer contact the Law Society of WA as they can refer you to a suitable lawyer in your area. I would recommend you seek a lawyer familiar with both family law and general land law disputes as your circumstances give rise to a number of different potential legal issues.
I trust the above assists.
Please rate my answer.