Polaczuk said the PSA understood there would be a reduction of around 25-30 per cent of staff by 2021.
IRD said there would be no reduction in the frontline services staff in the organisation. Commissioner Naomi Ferguson said 3300 customer-facing staff would all be "offered new roles or confirmed straight into new roles".
A spokesman for the government organ said those who are not confirmed straight into new roles may have to go through an application process.
IRD has also added 18 specialist positions across the organisation.
IRD shut its offices four hours early today and closed its call centre for more than two hours so staff could hear about changes at the organisation.
The government department, which employed 5789 people as at June last year, said members of the public could still sort their tax affairs online or use its automated phone service during this time.
"Inland Revenue is closing some services today so that all of its staff can hear about changes to the organisation. Contact centres will be closed from 12.45pm to 3.30pm. Offices will close from 1pm until the end of the day," IRD said in a statement today.
In March last year, IRD announced it would cut 1500 jobs "across the board" between 2018 and 2021. It said the workforce would shrink by 25-30 per cent.
The transformation plan is part of a $1 billion-plus modernisation of the tax system and the biggest change to the system in 25 years.
An IRD spokesman could not be immediately reached for comment.