Splitting a commercial/residential water bill Perth help?

Hi everyone we have a property that contains 3 shops and recently built another 3 offices/shops next to them with 3 residential apartments above. The buildings are not strata titled so we are getting one water bill for the entire plot which makes a little hard to split the bill amongst all the tenants.
Every building has its own sub meter so calculating water use is easy enough.
Are there any programmes/software available to do this what makes it hard is the sewerage volume charges the first 200kl of water there are no charges but after this they are around $3.4 a kl.
The shops that are tenanted have different water uses that are all over the place so wouldn't be fair to charge one rate for everyone any help appreciated?

 

@joca - You aren't able to charge the sewer discharge to residential tenants.

Waste water, is calculated in direct proportion to usage. So if a retailer uses 10% of the water, charge them 10% of the sewer costs.

Are there any common area water/waste water charges (again can't be charged to the residential tenants but % can be charged to commercial tenants if it is in the lease).

Don't forget to add GST (if you're registered) to the charges on the commercial tenants or you'll be out of pocket.

 

Thanks for reply there are no common areas looking at the last water bill the water use is easy enough as I have the sub-meters, there is one main meter so the service charge for this I split among all tenants equally the shops waste water sewerage charge is charged by the number of toilets (main fixture) as each shop has one toilet that's easy enough also where it gets a little technical is the first 200kl of sewerage there is no charge after 200kl there is sewerage volume charge per k/l.
The apartments, I live in one family lives in the other I actually just got an adjustment bill from the water corporation as these are new buildings that have just been occupied they are charging commercial rates on the apartments also that means each apartment gets charged for 2 toilets with the cost of this and the other water charges it makes the water bill of the residential apartments sky high. I will give them a ring and see what is going on, I can see why getting things strata titled so much easier.