The loss carry-back tax scheme announced in this week's budget is a "courageous" and "smart" move by Treasurer Josh Frydenberg to focus the economic recovery on businesses that have already proven their worth, according to leading accountants.

Under the $4.9 billion measure, companies with annual turnover under $5 billion will be able to offset losses incurred up to June 2022 against profits made during or after the 2019 financial year.