You shouldn't need to fast before an HbA1c test as it's an indication of your average glucose levels over the past 3 months and so any last minute fasting won't have any impact on the results... But maybe you're having another (fasting) test as well if you've been told to do so? However if this is the case, you should be OK anyway as there's nothing really in the water that could affect your glucose levels. As far as I'm aware the only time you may have to restrict water intake to small sips is before a general anaesthetic.
I have always been told by my doctor that I mustn't eat anything at least 12 hours before a test. The nurse asks whether I have eaten on drank anything but water before taking the test.
On my Pathology request form it says
'Fasting Instructions
You may be required to fast before having a blood test. This is because certain test values change following the ingestion of food. The measurement of blood sugar (glucose) and some blood lipids (fats and cholesterol) are common examples'
I would ring up and check as usually if you are having a fasting test - they would give you an appointment in the morning. Mostly for the test you are going for it would be non-fasting.
Yes, even though fasting (or not) isn't relevant for an hba1c, I always try for a morning appt and fast from the night before - because I always try and persuade the nurse to do a fasting cholesterol test at the same time.