I don't know Australian politicians at all, so not taking sides, but from $1BN spent on a stadium, one might hope some money will be recovered from tickets and sport events, plus you are left with some infrastructure, and at least the money are spent inside the country.
Quantum computing, on the other hand, so far proved to be only a buzzword and a media hype. Theoretical speculations and fragile toy-demos, with no hope for practical applicability. The field is stalled since decades. Then, the money are spent on a company outside Australia. That's a big red flag. If it were to be any breakthrough to justify the investment, that would be developed inside the country, and most probably behind closed doors.
Also 1 BN is a lot of money no matter for which government, would be a mistake to normalize the abnormal.