Only if you can show a population dense country like Japan spends comparative amounts on infrastructure to supply the gas than a population sparse country like Australia.
In other words, only if you're lying is it corruption. Costs are not like for like, not even close. The gas itself is relatively cheap. The infrastructure and OpEx is not.
I imagine this is usually because these countries sign long-term supply agreements at fixed prices. This is good because it guarantees stability of demand for companies, but obviously bad if gas prices go up after signing said agreement.
The future fund exists solely to pay public servant pensions and even then by it's own account it won't be enough to fund expected liabilities, the average person will never see a cent of it.
For example: Australian Gas consumers pay more to use natural gas in Australia, than people in Japan, using Australian LNG.