Very interesting, thanks Big T. I'm picking that most of the wrecks you describe long ago sunk into the mud... but the one in the Whau river is still there going by this satellite shot:
https://www.google.co.nz/maps/@-36.8898966,174.6682281,97m/data=!3m1!1e3
Be a good excursion when the tide is right. Don't fancy trying to drag a kayak through that muck