The problem is not with
OpenCPN. You have a detailed plan of 1:8000 inserted within a smaller scale chart of 1:90.000. The larger scale chart is of course more detailed and is probably based on a later and more detailed
survey.
The discrepancy of the 5 m contour is because the
charts are different, as issued by the AUS Hydrographic office.
This is quite common around the world when HOs are splicing together charts originating from different surveys made at widely different times using increasingly better
survey equipment.
In this case, sand drift may also be factor.
Thomas