As a few others have said having the aerial at that hight might not be the correct hight, I have seen a large aerial on a 2.5m mast ( not los ) get nothing, drop down to 1m and had so much signal we could use a smaller aerial.
There were some changes in the transmitter mux's recently so the TVNZ mux might be in the fringe of the bandwidth of the aerial, without seeing the polar plots it is hard to know if that is the current suitable aerial for your area.
I would try a large phased array aerial like this http://www.freeviewshop.co.nz/large-phased-array-aerial-freeview-p-595.html and a new piece of coax as an experiment, move it up and down the pole ( standing height ) to see how it goes, if it works clamp it on and enjoy, if it does not work you will have to try something else.
I have helped with 30ft masts in the past and know the issue of dropping them and putting them back up.
Without a decent signal strength meter you are really just guessing.
If gain is the real issue something like this will help.
I have not had a look at the polar plot to see if this is suitable.
A masthead amplifier will possibly create more issue, they are like wifi extenders, most people dont know how to install them and use them properly. if you have a crap signal going in, you are going to get a larger crap signal coming out.
Best of luck.