It will be a breach of the agreement and a potential offence under the Housing Act 2003 (Qld).
Per the Queensland Government website on public housing, the is a page titled "Changes to your household" (Changes to your household | Homes and housing | Queensland Government). This includes persons joining or leaving the household, changes in income, and visitors who stay longer than 4 weeks (who must be notified to the government). Failure to notify can attract a penalty of up to $1,305.50.
Subletting is also an offence under the Act, which attracts a penalty (also up to $1,305.50).
They can also recalculate the amount of rental assistance, and/or seek to terminate the tenancy agreement for breach. I'd say the fine and recalculation are the more likely options.