holy crap thats massive... must be a typo..... do you mean 0.5ppm or 5ppm of ammonia ???
in perth tap water generally comes with 1ppm of ammonia... 2ppm of chlorine and chloramine as well (i havent measured it yet as i dont think theres a test kit available for it) but to estimate chloramine is trial and error... air ammonia off the water over night and then add enough prime to break down the chloramine and retest for ammonia... trial and error stuff with no real accurate result but it has given me the knowledge of how much prime to add to neutralize chlorine and break chloramine bond and then furthur neutralize the released chlorine and ammonia....
i think the mundaring weir is the only water source that has chloramine in your water...
so to remove ammonia from your aging barrel simply add airstone just under the waters surface and have a small circulating pump in the bottom and usually 1ppm of ammonia will air off over night (12 hours)
test ammonia before and after you do this to confirm it for yourself