John - How tall is it? From what little can judge from the image it does not seem to me to be exceptional - for example, take a drive down to the mouth of the Wainuiomata River and you will see larger, taller trees than what I think the dimensions of your one appears to be. As far as I know you cannot easily age karaka because they don't produce annular growth rings. Someone else who knows more on this subject may be able to help you on that part of your question.
Ciao Peter