Here is the reason you need you cabin fire extinguisher re weighed
http://www.casa.gov.au/ADFiles/airgen/gen/GEN-065.pdf
And here is the reason it can be removed so the 6 month reweigh is no longer required
http://www.casa.gov.au/ADFiles/airgen/gen/GEN-065.pdf
Your aircraft is under 2750kg so not required to be installed IAW the above CASA AD, nice to have but at a cost. You could always have one in your flight bag which would not require a reweigh as it is not installed. Your AMO could have someone drive up and remove it, sign it off on the MR as removed, not required IAW the above AD.