Purely for information. My method:
Reduce each TIF to 8bit colour. I used Global Mapper.
Split files to North and South Island.
Merge with OziMapMerge v1.13 using Pixel Scale = 6.
Result = 2 .ozf3 images - NI 1.49GB & SI 1.87GB
Works perfectly in OziExplorer, Ozi Android and Androzic.
Edit - Scale = 6 (not 30 - that was for 250k maps).