Strangely high Mg readings - Low Ca and Kh though

[TURSTY]

Hi Guys, I put this post in my tank thread, but thought I would put up a seperate thread.

Ive been getting some strange readings from my red sea foundation kit which has me a little concerned.

I have done the Calcium, Magnesium and Alkalinity tests twice now and got the following readings: - Keeping in mind nothing in the tank has changed between these tests (no water changes and no suppliments have been added)

Test 1:

Salinity - 1.024

Ca - 375ppm

Mg - 1000ppm

Kh - 7.6dKH

Test 2:

Salinity - 1.024

Ca - 410ppm

Mg - 1600ppm

Kh - 7.3dKH

Test 3: (I just retested the Mg because I thought it seemed weird that the other two were so low and this was off the charts compared to the test a day earlier)

Salinity - 1.025

Ca - NA (didn't test it)

Mg - 1600ppm

Kh - NA (didn't test it)

I took a water sample down to the local pet shop and they did a calcium test on it and got 400ppm with a salinity of 1.027.

I also tested some freshly mixed water in my drum and it came back the same - Mg at 1600ppm

Is it most likely that my Mg test is not testing accurately, or is it possible to have a hugely high level of Mg in the tap water (wellington water supply)?