Anyone who doesn't get their 2018 tax assessment back this year will not be put on the default 21% rate from January.
Revenue Jersey says the rate will be based on earnings data from employers instead.
It's promised that 50,000 notices will be sent out by Tuesday 10th December.
Many are being sent late this year because of the time taken to move files onto a new computer system, so forms can be completed online from next year.
Each year, all ITIS forms are usually sent out by November.
Comptroller of Taxes Richard Summersgill says people won't be penalised because of the delay.
"We are doing our best to catch up with the late assessments, and we want to reassure taxpayers that they won’t be penalised for the impact of our modernisation programme.
"We are taking a flexible approach to applying late penalties this year and will spread any significant ITIS underpayments over next year, to avoid un-manageable increases in ITIS rates for our customers.
"I am sorry for the worry this has caused and I hope the approach we are taking will go some way towards reassuring people about their tax arrangements."
People who are self-employed, retired or have other non-employment income have to make their second payment on account, for the balance of their 2018 tax bill, by 6 December.
However as many people haven't got their 2018 assessments back yet, Revenue Jersey says anyone who didn't get an assessment by 25 October can use a best estimate to make a payment by the end of December 2019. Alternatively, they can wait for their tax assessment and pay the balance then.
The concerns prompted a written question from Deputy Kevin Pamplin - with the answer revealing that more than 20,000 people were still waiting as of 4 November.
TAX FIGURES: Further to the Ministers response to my oral question in the @StatesAssembly this morning, here are answers to my written answer on the numbers & info on the ITIS delays. 20,600 islanders are still waiting for their updated rate notices 🇯🇪👇🏼 pic.twitter.com/0NGmylTlvB— Deputy Kevin Pamplin (@KevinPamplin) November 12, 2019
There's more information here.