Doesn't help in this case, but I do love my $49 LG Optimus Spirit, unlocked for $2 and then just buy a sim in-country.
Bugger buying on plans, being an older guy who probably has lower usage, the Aldi PAYG with 365 days of credit would probably be far more suitable.
https://www.aldimobile.com.au/plans#payg
Can't buy pre-paid sims in Japan. you know because terrorists might use them or something