I like to use SkyScanner and find the "Chart" showing the price variance each day of the month very handy.
Ie Tokyo-Narita: https://www.skyscanner.com.au/transport/flights/syd/nrt/cheap-flights-from-sydney-kingsford-smith-to-tokyo-narita.html?adults=1&children=0&adultsv2=1&childrenv2&infants=0&cabinclass=economy&rtn=1&preferdirects=false&outboundaltsenabled=false&inboundaltsenabled=false&oym=1702&iym=1702&selectedoday=01&selectediday=01
Note: it's worth checking all international ports in Japan in case the fares to those other than Narita are cheaper
Ie Haneda: https://www.skyscanner.com.au/transport/flights/syd/hnd/cheap-flights-from-sydney-kingsford-smith-to-tokyo-haneda.html?adults=1&children=0&adultsv2=1&childrenv2&infants=0&cabinclass=economy&rtn=1&preferdirects=false&outboundaltsenabled=false&inboundaltsenabled=false&oym=1702&iym=1702&selectedoday=01&selectediday=01