I visited in May, mid-week. The City was full of tourists so possibly this was just a busy day at the Opera House. We purchased tickets for the Tour in advance, which enabled us to skip the box office but we still had to wait in the same line as those still needing to purchase their ticket. When we finally made our way up to the box office, they let us into the building but we still ended up waiting for 45 minutes with the general public as they only have one security screening. But well worth our time. Luckily, our tour group waited in the lobby for us.
You choose your time of visit so no need to skip the line. Come 15 minutes before.