Under the Strata Schemes Management Act, an Executive Committee and/or Managing Agent has no discretion to relieve any owner of their obligation to pay interest on overdue levies, which is prescribed at 10% per annum. Relief from paying interest can only be granted by passing a Special Resolution at a General Meeting of all the owners. You are correct in that it isn’t fair to the other owners.
You are entitled to see the records of these payment plans. I’d be after the Managing Agent’s scalp, and would be retrospectively charging interest on the plans which are invalid, and follow up with debt recovery action where unpaid. Suggest get a new managing Agent pronto.
David