Cowboy is quoting figures that do not apply to a by election in a labor held seat when there is a labor govt in Canberra. He is using average swings across all by elections. But that is not applicable to a labor seat in a labor govt.
The correct swing has been 8.2% against Labor according to Antony Green.
This is direct quote from Green.
At the eight by-elections in Labor seats during Labor governments, the average swing was 8.2%