It was Tom Springfield. One way to find this out is as follows:
1. Enter "Island of Dreams" in Google. 2. Observe from the results
of 1. that it was performed by Dusty Springfield and by the
Seekers. 3. Enter "Island of Dreams Dusty Springfield" in Google.
4. The first result of 3. is the lyrics. Click on this link. 5. The
lyrics contain the author Tom Springfield. 6. Check that it was Tom
Springfield: Enter "Tom Springfield" in Google. The Wikipedia entry
for Tom Springfield says that he wrote "Island of Dreams". Other
background is that both Dusty Springfield and the Seekers had an
artistic connexion with Tom Sprinfield. Answer provided by "The
long distance pop music worshipper".