The Full Federal Court in E I Du Pont de Nemours & Co v Imperial Chemical Industries PLC  FCAFC 163 has ruled that just because a Parent Patent was found invalid due to lack of novelty, it did not follow that a Divisional Patent based on the Parent was also invalid.
For those of you who might not be familiar with Divisional Patents, here's a quick lesson. Often, while a patent application is still pending, IP Australia may reject the application because it attempts to cover or define two inventions in the claims. The applicant will then have the option to delete the offending claims and to file a Divisional Patent Application with the deleted claims. In effect, the patent application is divided into a Parent and a Divisional. There are a number of other situations that can give rise to Divisionals, but I won't discuss them here.
In this case, Du Pont had been successful in opposing ICI's Parent Application which had been found to lack novelty. However, Du Pont had previously filed a Divisional Patent Application based on the opposed Parent. Du Pont argued that because the Parent Application had been successfully opposed, issue estoppel should apply effectively finding that the Commissioner of Patent should not have accepted the Divisional for grant.
In DSV Silo-und Verwaltungsgesellschaft mbH v Owners of The Sennar  2 All ER 104 Lord Brandon formulated the doctrine of issue estoppel as follows:
"...in order to create an estoppel of that kind... three requirements have to be satisfied. The first requirement is that the judgment in the earlier action relied on as creating an estoppel must be (a) of a court of competent jurisdiction, (b) final and conclusive and (c) on the merits. The second requirement is that the parties... in the earlier action relied on as creating an estoppel and those in the later action in which that estoppel is raised as a bar must be the same. The third requirement is that the issue in the later action in which the estoppel is raised as a bar must be the same issue as that decided by the judgment in the earlier action."
In this particular case, the Full Court found that the third requirement was not met.