Hi there, hairdresser here - LOL!!
Now, firstly I need to ask are you sure that your hair is nearly black?? The only reason I ask this is often when clients ask to go back to their natural colour, it is a LOT darker than they remember it being - and even in saying this your hairdresser should have discussed this with you and shown you a swatch of a colour very similar to what yours is against your face and with all your other hair pulled back so you could get a 'feel' for it first.
Also, how light was your hair before going darker? Was your hair so light that it needed to be filled first - as if this was the case your hair would have been more likely to 'grab' the colour and will also be more likely to fade off faster (again, this should all have been discussed with you).
Short answer - yes, you can have it coloured lighter, but your hair will certainly feel the effects of this as it will essentially need to be stripped to some degree... so I would be inclined to contact the hairdresser who originally coloured it first to find out exactly what has been done and if it will fade off quickly etc...
I do think the inital problem stemmed from lack of consultation with you in the beginning though, so there is no need to feel embarassed or insecure to tell your hairdresser how you are feeling.
PM me if you need any other help - I am a trained specialised colourist, but unfortunately not in Sydney so cannot help you out in that regard

Good luck!