So I'm planning to post something overseas and Australia Post suggested 10 maximum working days.
Is that the day I post the parcel + 10 days? or the day I post the parcel + 10 days - weekends (if the parcel is dispatched from Australia on Friday morning, the postage times will start on Monday-Friday then Monday-Friday = 2 weeks)?

If you post the parcel on a Monday, say the 1st, then it will be delivered by Monday, 15th, at the latest.