In Australia, teachers (like many workers in unionised occupations) are paid in accordance to an "industrial award". The awards are determined by state.
To support that with an example, the Queensland government says:
Teaching salaries in the [Department of Education and Training] are based on the current awards and agreements.
It is over-simplifying a little, but yes, for Queensland where Kathy Margolis was based, the claim is true.
The Teacher's Award for the state of Queensland explains:
6.1 Hours of duty - classroom teachers
6.1.1 What is the rostered duty time of a teacher?
(a) The rostered duty time of a teacher will be 25 hours per week.
(b) Rostered duty time will be continuous except for the meal break.
Rostered duty time includes "face to face teaching and associated professional duties" and "preparation and correction time" (See Section 6.2), and some short breaks (See Section 6.1.3)
However, there are other sorts of teachers included in the award, such as "instrumental music instructors", "education officer special duties", and "teachers in centres for continuing secondary education/secondary colleges" that have different rostered duty times.
The Western Australia Department of Education has the Teachers (Public Sector Primary and Secondary Education) Award 1993
Section 13 (4) says
Effective from the commencement of the 2011 school year, the maximum number of hours of face to face teaching is as follows:
(a) Secondary – 21 hours and 20 minutes per week
(b) Primary – 21 hours and 50 minutes per week
(c) Pre-Primary – 21 hours and 20 minutes per week
In Section 14 (2) (b) it adds non-face-to-face time:
Effective from the commencement of the 2011 school year and in addition to face to face teaching hours, the following periods of time (exclusive of recess and lunchtime) are provided
(i) Secondary – 320 minutes per week
(ii) Primary – 240 minutes per week
(iii) Pre-Primary – 320 minutes per week
New South Wales
The NSW Department of Education has the Crown Employees (Teachers in Schools and Related Employeers) Salaries and Conditions Award 2014
However, there isn't a simple count of the number of hours in this document - it is based on 28 x 40 minute teaching periods per week in high schools, plus additional alternate periods, and non-teaching time. I am loathe to do original research to compute the hours; there is a lot of complexity here.