Centrelink Debt! Advice Please!?

Long story short.... I work full time and my partner works part time whilst staying at home with our 2 year old son during the day. When we first applied for Centrelink benefits we told them what my wage would be every week and what my partners wage would be as these wouldnt change a cent at any time. This was about 4 months ago and today we were contacted by Centrelink to say that we owed them thousands of dollars because they didnt have my partner down as working anymore. They said they have a record of him calling to report he had a job and what his income would be and then for sone reason that status was deleted. There is no record to say he contacted them to say he wasnt working anymore, a staff member there simply deleted it for some stupid reason!! The guy on the phone was quite nice and said he didnt understand why that had happened. Now we have to wait up to a month for them to tell us exactly how much we owe. Do we have grounds for appeal and if so, how long and tedious is a fight with Centrelink? Has anyone else ever taken them on and won/lost? Im so mad right now!! Grrrr