Cancel Koseki to get the CoE for a LTR?

Hi, I'm a 2nd generation Japanese who was born in Australia and is now applying for the CoE in preparation for the a Long term visa application. My mother was a Japanese national when I was born and submitted the details to Japanese immigration. She unfortunately gave up Japanese citizenship afterward (she and I have Australian citizenship) and is now living in Japan on a long term visa. She's quite old now and I have decided to retire and look after her.
I have submitted all the paperwork (including both Koseki and Australian birth certificate) for the CoE but have been told that I need to either cancel the Koseki (which I am hesitant to do) or give up my Australian passport which I assume means getting a Japanese passport. I have been told by immigration that the CoE would be granted once I send them a shomei sho from the local council office of the cancellation.
Can anyone confirm what I have been told by immigration concerning the need to cancel a Koseki is correct? My Japanese is poor and when I spoke to immigration I am unsure of what they have told me so I want to be sure before I go and cancel the Koseki, Thank you.

I do believe that important questions, where you have to make decisions are better to be asked with an immigration lawyer, or ask (if there is) a family member who can speaks Japanese and contact the immigration or visit the immigration. Also you can go to the Japanese Embassy maybe they can advise you also.

Dedicated manners like this on forums will just give you more different opinions.

Not a lawyer here, but if you still have yourself in gkoseki,h that means you still are a Japanese national? Then you hold gdual citizenship,h which the Japanese government does not allow, so they are asking you to give up the Japanese citizenship in order for you to get a Long-term Resident status as an Australian citizen.

Since your mother has been living in Japan as a long-term resident, she must have removed herself from the Japanese gkoseki,h so they are asking you to do the same.

You probably submitted the gkosekih (probably with the description of the facts that she IS your mother, that she later gave up Japanese citizenship, and that you havenft) as a proof of your relationship to her, but it also served (to the Japanese authorities) as a proof that you have not given up your Japanese citizenship, if you still have it, that is.

Had you known about the situation a long time ago and gotten yourself a Japanese passport (based on the gkosekih) and entered and left Japan using that, and entered and left Australia using your Australian passport, that would have been ok (or tolerated), but now that the Japanese government is fully aware that you are a dual national, what they are asking for seems to be the only option for you to proceed.

I would find an immigration lawyer, though, to seek their opinion.

ask an immigration lawyer.
but, I am afraid that you are violating the law about koseki. (i.e. you don't report your current situation that you already lost your Japanese nationality.)
they have kindly suggested you to correct your legal situation by yourself.

You're a Japanese citizen. Visas are for foreigners. If you want a visa for Japan then you need to renounce your Japanese citizenship before applying for it.

The easier thing to do is to get a Japanese passport. No, you don't need to give up your Australian citizenship to do that. Just go apply for it.