sounds perfectly normal to me hun

what has happened is you either overestimated your income signficantly or opted not to receive part of your centrelink FTB payments each fortnight

Arrears payment of Family Tax Benefit Part A Supplement this is part of your annual payment that is held back until after your taxes are done. if you have estimated your income correctly, or overestimated, you get the full entitlement (about 686 per child unless born in that fin yr, then it's less) - if you underestimated your income, some of this is taken to repay any debts
Arrears payment of Family Tax Benefit Part A you overestimated income so you get more of your FTB that you should have been getting each fortnight but didn't. this is based on combined income
Arrears payment of Family Tax Benefit Part B same as FTBA - this is based only on the lower income earner - so if you said your income was 10k but only 5k you get more of a top up
Arrears payment of Rent Assistance same as FTBA - rent assistance is paid to "low income" families and the amount is based on your annual income estimate. if you overestimate, you get more rent assistance
Arrears payment of Family Tax Benefit Part B Supplement same as FTBA supplement


sounds totally normal to me - ENJOY!