The NDP is warning Ontario parents the new harmonized sales tax may be applied to a crucial but costly therapy for autistic children.

NDP Leader Andrea Horwath is demanding the government come clean about whether it will apply the tax on intensive behavioural intervention, a therapy known as IBI.

Horwath said at a news conference Tuesday that the government is anticipating the HST will apply in some cases where the treatment is used.

She said tax may not apply if the psychologist providing the therapy is exempt and included in a national registry of psychologists. But the government removed that requirement for patients who want to obtain IBI for their kids.

Horwath says the HST move rubs salt in the wound for parents who are already dealing with long wait lists for the treatment.