The Tax Office cannot guarantee it will begin tax time 2017 on July 1 as it scrambles desperately to save this year's tax return program from the fall-out of its disastrous pre-Christmas online meltdown.

The agency has abandoned much of its IT program for this year and can only say it is "confident" that businesses and taxpayers will be able to lodge and receive tax returns on the day after the end of the financial year.

ATO chief information officer Ramez Katf.

ATO insiders believe the office should be able, just, to avoid having to postpone tax time 2017 but it will come at the price of much of its technological program for the year.

In the wake of the recent tech failures, the peak tax accountancy body, the Tax Institute, said on Tuesday that its members had lost confidence in the ATO's systems.