I have just installed a hardwired  inline filter.  It made a difference of 5Mb/s (11.8 to 16.8).  I had tried the disconnect all your phones test prior to installing the filter but got very little improvement.  I realise now that the house alarm is hardwired into the phone line prior to anything else and that possibly had a impact.
The filter is now installed prior to the alarm.
I notice most peoples speedtest.net posts are for the Auckland server.  I am getting better speeds and shorter ping times using others such as Tauranga (Auckland server is closest).