That makes no sense.Can't imagine the unions being impressed. They are effectively out of a job, or more accurately a function.
In the implementation phase, employers are required to cut wages by $X per week across all employment sectors; Employee total income on week one of UBI should be equal to their total income the previous week, the difference being that now the employer only pays the amount above $X.
For employees whose incomes are below $X, the employer and employee would negotiate a new pay structure (which might be "tips only", or a small wage, or the elimination of the job altogether; or any other mutually acceptable arrangement).
Future wage and salary levels are purely a matter between employers and employees; Raises and/or pay cuts by mutual agreement. If employees are unhappy, they can quit; If employers are unhappy, they can fire.
Since you are an Aussie you would know how wedded we are to Enterprise Agreements and workplace bargaining.
Employees would still benefit from collective bargaining and unions would still be the best way for workers to organise.
Agreements between employees and management would still be formalised in EAs. UBI would only change the pay levels in these agreements.
Besides, unions also bargain for a whole range of other conditions besides pay.
A lot of smaller workplaces do not have EAs and their employees are not in a union. These employers are currently required to pay wages according to industrial awards, which specify schedules of wages based on job classifications.
If we implemented a UBI we might scrap the pay provisions in these industrial awards, and leave it up to employees in small shops/offices to negotiate better pay commensurate with their level of responsibility and skill. But some employees would definitely suffer: not everybody is good at negotiating.
I think we would need to keep some kind of pay provisions in industrial award or in the national employment standards, which require all employers to give their workers automatic pay rises at the beginning of each financial year to at least keep pace with inflation. This is a critical feature of awards and ensures that employees aren't forced to negotiate each year just to avoid a cut in real wages.