We only ever do a lump sump claim at the end of the financial year as mine & DH income changes fortnightly by hundreds so I never claim fortnightly so I don't risk raising a debt with FAO.
On the Centrelink website you can fill out a form to make a claim on the last financial year. It wouldn't hurt to give them a call as you may be able to lodge a claim for the last two years.
I found out last year when I did my claim that there is a large family supplement once you have 3 kids that you may be entitled to a portion of.
good luck.