Your employer seems to have no idea.
My bet is that they just don't know and it's an honest mistake.
I had to educate my payroll people on a couple of things and my company ALWAYS get your pay wrong when on maternity/parental leave.
Could you send your work a copy of the fair work provision for LSL so they have it in writing?
You're entitled to your LSL. Don't fret, stay calm and to the point, keep referring back to the legislation and I'm sure it will be sorted soon