The information was a mixed bag of facts and opinions, reddit style, but there are some people on there who are pretty knowledgeable on the subject so it’s worth a look. Some of the chat on there in combination with my own research kinda got me where I needed to go. There’s also a lot of good financial advice on the RetireJapan.com forum.
For my situation it turned out that what I would need is a tax advisor in the UK rather than Japan.
It appears that from the Japan side; things are pretty straightforward. In simple terms I would declare my earnings from my job as Overseas Income on an annual tax return and would then be taxed on them accordingly.
The complications lie more in the UK (surprise surprise). I can continue to earn and have my earnings visible to the tax office in the UK and my employer would need to issue me with an annual document to state that my tax affairs in the UK are in order. I would likely be entitled to have most or all of that income tax rebated at some point as Foreign Tax Relief, depending on how my activities compare against a set of thresholds (hours worked in UK/Japan, time spent in either country, etc) in any given year.
I would sever residency in the UK on the date that I leave. This would trigger the tax office to instruct my employer to update my tax code eventually and after that point it’s unlikely that I would be paying income tax in the UK on my earnings (or at least be paying at a lower rate) but it would likely take a year or two for things to settle to the point where I’m able to just pay my taxes in Japan without chasing rebates in the UK.
I don’t know how this would differ in Australia but the resounding message was to seek an accountant/advisor in the country where I’m earning the money because that’s where the complications would arise from.
Plot twist; for all my ongoing efforts in getting informed on these matters my employer in the UK announced at the beginning of this week that they would be “letting me go” at a date that coincides with my leaving the country (as part of a round of redundancies) so I will be looking for a job in Japan after we arrive. Mixed blessing, I think; I’m gutted to be leaving the job as it’s somewhere I’ve really enjoyed working but I’m glad to not have to deal with the hassle of managing the dual tax situation on top of everything else (flights, visas, house hunting, school application, nursery application, container shipping etc.). Looking forward to a new start.
I realise that descended into an anecdotal ramble - hopefully there was something of use in there.