Welcome to the forum. Those 15L ones from Bunnings are the go, you bring in the old containers and swap them whilst you are eating the sausage sizzle. That way the price is further discounted.Now that I look on bunnings website, I can see liquid chlorine available in 15L for way cheaper... good tip for next time
I'm a new pool owner too. All a bit overwhelming at first but the folks on here and the pool school book will keep you on the right track.