The theory of water injection into turbine (& piston) engs is that after compression (or supercharger) the air is hot - bloody hot actually, well over 250*C in JT9D. So, water is injected into the diffuser case where it vaporises, extracting heat from the air. Cooler air = more dense air = greater mass/unit volume (recall the compressor pumps volume, not mass) hence more mass flow and the ability to burn more fuel without a meltdown.
On the JT9D it was 600Kg/eng in 2.5 min. Felt nasty when the water ran out and the old bird died in the arse on a 38*C night ex BKK at about 1000ft. Only tried it with t/o eng fail once - very, very nasty!!! P&W went for demineralised water while other eng mfrs used water/methanol mix which gave a slight performance increase, but is diabolical to store and handle safely. Only used for t/o on JT9D. Other engs, eg TPE331 in Metro 23, also need it to satisfy ldg clb performance on 1 eng.
To answer the question, Qantas has no water inj capability on any of its B747 fleet. As I understand it, neither does Ansett.
As may be readily imagined, water inj is a fiddle which takes the engine very close to its temp limits, and a slight malfunction can be spectacular with a large shower of assorted nuts and bolts if it goes haywire - a blocked/distorted water inj nozzle will do it. Given that it was only ever used when weight limited your heart rate was always somewhat raised until the water ran out as you climbed away.