Tony. NiCad or NiMh batteries are both 1.2 volts nominal. They are considered 'dead' at 1.0/1.1 volts, but straight off the charger can be anything up to 8.4/8.7 volts which slowly reduces. (6 X 1.2 = 7.2 volts, 6 X 1.45 = 8.7 volts).
It depends entirely upon what the manufacturer has set the charger cut off voltage at. Normally around 1.4 volts per cell, which gives you 8.4 volts.
(I state 1.45 volts because that is/was considered as the fully charged battery cutoff point).
If you are going to buy/use any, 240 volt or 12 volt to 9/7.5/6/4.5/3 volt power supply, then please make sure it is a 'Switch Mode' or regulated one.
The 'normal' (insert cheaper here), tranformer type '240 to 12/9/etc, plug packs' are not regulated & can put out a lot more voltage at low loads than you require. I've seen/measured some at up to ~17/20 + volts when supposedly putting out just 12 volts on 'no load'. The lower settings also show this tendency.
(Must Be Switch Mode/Regulated).
This one is a 240 volt plug pack from Jaycar > #MP-3031 (Adj Volt @ 1.25 Amps), @ $45.
OR this one is Dc to Dc converter #MP-3014 (Adj volt @ 1.5 Amps), @ $24.95.
This is from a 2005 cataldog. Might have changed since.
HTH.
L.