Invoice for water usage

It is high, but not unusually high. The average Belgian home for 2 people uses 70m3. (See here: https://www.aquawal.be/fr/consommation-d-eau.html?IDC=528 )

You can also use their on line tool to calculate what your usage should be (see here : https://www.swde.be/fr/infos-conseils/simulateur-de-consommation )

The cost per m3 as well as the network costs will be shown on your invoice.

The difference over the average could easily be explained if you take baths rather than showers. Even the use of a power shower will push up usage. It is also possible that the period between meter readings was more than 12 months - the dates used for the calculation should be shown on the invoice - if this year's reading was late it should more or less self-correct next year.

Yep it's possible. Prices are online for your water company. The more you use, the higher the price per m3.

We are 6, average over 12 months is 140m3 so 24m3 each, you use almost twice per person.

And if you're in flanders, water is more expensive than other regions.

Look at your bill. It will show cost per m3 and how many m3 at each level. Average m3 isn't going to be helpful, finding out when it becomes more expensive per m3 is, then use less water to get to lower cost level.

I live alone in Flanders and my bill comes from De Watergroep. For the year ending in February 2018 my bill - one person - was €175.00.
I am meticulously careful over my usage. Some years ago I realised that, in the months when the heating is off, 5 litres of cold water goes down the drain every time I need hot water. I now use those 5 litres for watering the plants, the washing machine and so on and I reduced my bill significantly.
If you have a bill for the whole year, I'm guessing that it is your first bill and so they have nothing on which to base an estimate. In future you will get a quarterly bill which will be based on your historical use and an annual bill based on your actual usage less the 3 quarters which you have already paid.
If my guess is correct, the obvious thing to check is who took the readings at the start and end of the year and whether they were accurate. If you have accepted an inaccurate reading at the start, sadly you are stuck with it.
Check you meter regularly and, if you find that you are not using anything like the amount that a €1200.00 bill implies, contact the supplier and ask them to reduce your future quarterly bills.