I found that I was drinking lots when it was really hot in Adelaide recently. Of course I was walking about in the heat. I was drinking 3-4L at least.

THis is a guess but I think your fluid requirements increase in pregnancy, particularly in the later stages - you have extra blood volume to maintain, plus all the other fluids. If as you said it has been hot the thirst is probably your body making sure you get the fluid you need.

If you are really concerned seek a medical opinion.