That depends on what gauge your incoming supply cable is.

If your supply can support it, swapping it out isn't that big a deal. (Note : Do not try if you have no experience or qualifications with power systems)

3 air con, 2 fridges and 2 water heaters definitely will tarpau your supply and set your house on fire if you happened to run all of them at the same time lol. I don't even need the AS3000 standard to tell me that.

Might I suggest going to 3 phase power instead? Split the phases and balance the load correctly.

12kW would be around 50+ amps which is way over what the protection device can handle at 32 amps.

Proper cabling is important, but you'll have to size your protection (be it circuit breaker or RCD correctly), also water heaters should always have an RCD instead of a standard SP/DP circuit breaker to prevent death by electrocution. The last thing I want to do is die naked.

One should also note the size of ground wiring and staking and resistance required for the load being used. If you supply cabling is 20 mil and can carry 150 amps oso no use if you neutral conductor is small and your neutral block is tiny.

Electricity isn't just about current.

He should survey what kind of power is coming in before shoving 2x 32 amp boxes in there. A little bit of homework saves a lot of issues in the long run. On many industrial 3 phase applications I've done, we sometimes keep the neutral skinny if we are connecting to a well balanced load to save costs as allowable by AS3000 or other relevant standards.

You don't need the same size of neutral conductor for a 3 phase because Wye 3 phase systems are inherently balanced. Thus if you correctly balance a 3 phase system you'll never need more than the sum of 1 output conductor (actually you won't even need a neutral).

That's why TS should get a proper electrician to survey everything. A good electrician will figure the best way to balance everything. You'll never get perfect balance, but getting close is best. Also an electrician can see if anyone has daisy chained the circuit which is very common in houses, whereby instead of wiring in new cables, people just daisy it off the nearest circuit. If you're daisying off an already heavily loaded circuit, add something like a fridge will create problems.