Jetstar has some solid prices on at present on their sale which ends on Monday, and one of the highlights might be this - Cairns to Osaka from $350 return.
I did a play around and for flights going there there were plenty of availability for $189 one way, and coming back for $161. Total = $350^.
Sample date: Jan 20 to Feb 24 (note, Jetstar lets you see the calendar months so it's very easy to search for the date you want)
Note that bags/food are not included, but I took carry-on only last time in Japan and managed just fine.
^ For bookings in AUD, a Payment Fee applies except for POLi, voucher, gift card, Points Plus Pay – Flights (where the total booking is redeemed in Qantas Points) or Jetstar Mastercard. Payment options and fees may differ for other currencies.