That is correct.If you accepted the MIN offer technically you...

That is correct.
If you accepted the MIN offer technically you sold your NWE shares.
if you sold on market, you receive cash.
In this case you accepted the offer, thereby selling your shares and received MIN shares instead of cash.

This means you have created a CGT event and have to pay CGT on the consideration.
The consideration is the value of the MIN shares at the time you accepted the offer.
This is not the date you received your MIN shares, but the time you accepted the offer.

If MIN get to 80% there are special provisions in the tax act whereby you can elect for rollover relief from the CGT.
This means your CGT status from NWE now applies to MIN.
Also, your purchase price for NWE becomes the purchase price for MIN.
Effectively, everyone pretends your NWE shares were always MIN shares and you pay no CGT until you sell MIN.

If MIN don't get to 80%, no special provisions and you are liable to pay CGT/tax on accepting the MIN offer on the date you accepted the offer.