PAYG doubled this pay

We have one employee whose weekly PAYG has doubled from around $200 last week (and many weeks before that) to $400 this week. I also tried another payroll, without recording it, and it is calculating the higher $400 tax again.  He is a Working Holiday Maker, we have his TFN recorded and nothing has changed in the last week, apart from his take home pay reducing by $200, which he noticed and then checked his payslip, and came to me.  The tax rate shouldn't have changed - especially halfway through Feb.  Any ideas?  He needs that $200 to live on, and while I can manually reduce his tax each week, I would like to know why this happened.

Thanks,

Kim