Just noticed this deal is available again.
Now Australia wide, but still quite scattered across stores.
Stock checker, https://nrby.in/bunnings/0417097
I/N: 0417097
Ozito PXC 18V 2 x 4.0Ah Batteries & Multi Charger Pack $99.98
While only 2 batteries, this may suit some as it includes a dual battery fast charger.
Again, now Australia wide. but still scattered across stores..
Stock checker. https://nrby.in/bunnings/0370066
I/N: 0370066
As stumo said above, it's primarily the amperage (current) boost which provides the power, from the equation P=E*I (power, voltage, amperage) which you obviously know. The voltage remains the same.
Some Ozito batteries can be switched to higher Ah eg 4-6Ah or 5 to 8Ah, which is their way of upping the amps delivered. Of course the more amps/power you use the shorter the runtime.