Howdy Richard!
Good to see another aussie on the forums!
This increase in water is nothing to worry about, the recommended daily intake for humans is 8 glasses of water spread out over the day (actually a false statistic, but doctors do admit that this is healthy), but I know personally that in the hotter weather (and colder!) I'll drink above that.
As with any nutritional or medical problem, it is much easier to think of the patient as a machine rather than as a living being (However in theory life is just a complex organic machine!).
Rabbits, like all mammals have to maintain an average body temperature, if their temperature drops too low, their body's start to slow down and they will die! The same applies to if they get too hot!
This heat has to be generated (and removed) somewhere! In the colder weather such as we are experiencing at the moment (brrr it was 1 degree here last night and I'm just above sydney on the coast!), rabbits need to pump more fuel in to their furnace to keep them going. More fuel (food!) means more water is needed (this is also fuel but serves many other purposes), as the weather gets cooler your rabbits will drink more throughout the day and eat more food to keep their bodies running at the right temperature.
The increase in hay will also act as a contributing factor as Mrs. PBJ has pointed out, these things combined will act as major factors.
The same will apply in the summer, as the weather heats up your rabbits will need more water and you will see their intake spike above the so called 'recommended' levels.
I've seen Lucy put back a whole bottle of water in one day, then only sip at it for a whole week at other times.
It is always good though to measure your rabbits output and make sure it roughly coincides with the input. If you are seeing a big increase in water consumption and a big decrease in the rabbits urine then there could be a major problem.
I like to give Lucy perhaps a 3/8ths of a cup of pellets on a cold winter night instead of 2/8ths, it seems like a small amount but for their body size it is fairly big (like having a salad with your main course). I provide her with unlimited hay (which your boys and girls should be getting anyway) and unlimited water. You need to trust in your rabbit to keep it's intake and output equal.
Also at your rabbits current age they should not be getting more than a 1/4 of a cup of pellets along with unlimited hay. Try to mix your hay and your pellets up a bit too, it promotes good digestion to have a mix of differen't hays and grasses (i.e. give them timothy pellets and alfalfa hay, or orchard grass and timothy pellets.).
However everyone has their own opinions as to what rabbits should be fed the general consensus is that 1/4 cup along with unlimited hay is healthy.