This is what I got from the FAQ section of the compare benefits page (Between Points Club and Points Club Plus).
Once I'm in Points Club, how long will I enjoy the benefits for?
You can enjoy your membership benefits from the moment you unlock Points Club, until the end of your next Membership Year. So, for example, if your Membership Year begins on 1 January 2020 and ends 31 December 2020, and you unlock Points Club or Points Club Plus in June 2020, you can enjoy your membership benefits until 31 December 2021. To enjoy your benefits for longer, just earn 150,000 or 350,000 points each Membership Year to retain Points Club or Points Club Plus.
Despite the potentially clumsy wording, I think it infers you get the lounge passes at the time of earning/posting of points once per membership year. Other benefits eg 5% discount on reward store, SC on reward flights you can potentially benefit from for more than 12 months.