Does anyone knows how to defend yourself against SPARK charge for an internet speed check-up "It's not our fault" service?

Just hate it, especially after finding that someone else was charged in the same way:
http://www.geekzone.co.nz/forums.asp?forumid=39&topicid=173348&page_no=1#1308402

Our residential ADSL broadband was always running about 15Mbps download, but fell down to 9-10Mbps since 4th of April (East Auckland
area).
While trying to update all on our site starting from network card drivers and finishing with the ADSL modem firmware update, it was
decided to call Spark support.

I've literally spent hours of calling the support line, including waiting time and explaining my story again and again to every
random phone assistant, but their only answers were standard bla bla, like: "Did you check this? Did you do that? Yes, we can
see the history of your connection is down from 15Mbps to 10Mpbs since April 4th, but it's not our fault, please check on your end
or book our technician to come and check your residential connections, but beware of $150 extra charge if he don't find any faults."

"Well thanks, but could you just recover my original ADSL speed please? -No, we can't do anything from our end, or you can also
switch to VDSL/UFB if your speed is not suitable".
During the whole April and half of May is stayed 10Mbps and by making another call to Spark again, their phone assistant noted, that
technician already visited our place on Sunday morning, but no one was at home and he didn't find any fault". -Excuse me? Nobody
even call my number or door bell, but whatever.

I didn't paid a lot of attention to that note, as I was already sick and tired by fighting for these extra 5Mpbs on every day and
believed, that the technician onsite check could be a part of their standard service as I never asked for his visit, but only asked
to check the outside wiring cabinet or whatever else they can do check between the house and their terminal.
I knew for sure, that it wasn't our house problem, as we didn't add any new phones or internet devices during that time.

Little later there was a major power cut for an hour or so in the area, so it could be just a coincidence, but the ADSL speed
switched back to the previous norm of 15Mbps download.
Finally solved, but unfortunately, it wasn't an end of story. Few days ago I've received $150 bill for that "check-up service" done in May.

This is just great, 1.5 months of faulty service, plenty of wasted hours to tweak and upgrade every Internet device or calling Spark
phone support, but wait, you still own some money after all.
Nice job SPARK and very nice start track of young brand "reputation management".