You probably need to write a variable that will format the Date into Australia format using the function ToFormattedText([Date Field],"DDMMYYYY). Then "encode" the variable in your API call.
ToFormattedText (Date d, Text f)
Description: Returns a Text value containing the formatted print representation of the Date d, using the format f.
Example: ToFormattedText(Date(2000,01,30),"MMDDYYYY") returns "01-30-2000"
ToFormattedText(Date(2000,01,30),"MMDDYY") returns "01-30-00"
ToFormattedText(Date(2000,01,30),"DDMMYYYY") returns "30-01-2000"
ToFormattedText(Date(2000,01,30),"DDMMYY") returns "30-01-00"
ToFormattedText(Date(2000,01,30),"YYYYMMDD") returns "2000-01-30