I'm an Australian citizen with an international driving license and want to insure my auntie's car which I'll be driving in Poland for 7 days from 18 August to 24 August 2019. There will be 4 passengers in the car. My aunty has basic insurance but as I'll be driving I want to make sure the car is protected. I keep googling for polish car insurance companies but I haven't gotten any straightforward answers.
So the car already has Polish OC but you want to add short-term AC? Some insurers may allow you to buy one-month AC, but the condition may be that the car is OC insured with them. Hence, your best chance is the current OC insurer. Check this article: azpolisa.pl/Ubezpieczenia/OC-na-miesiac-AC-na-miesiac-Krotkoterminowe-polisy.html
I'd go with the basic insurance; for 7 days you'd most likely have to pay like for a monthly-rate anyway (which would be more expensive than if bought a 6 or 12-month term).