Question: Mention the series code of countries using Australian dollar as their currency unit. Which country belongs to middle income group among them.
SQL: SELECT T1.CountryCode, T2.SeriesCode FROM Country AS T1 INNER JOIN CountryNotes AS T2 ON T1.CountryCode = T2.Countrycode WHERE T1.CurrencyUnit = 'Australian dollar' AND T1.IncomeGroup = 'Lower middle income'