The aged care minister wasn't told about revelations his department might have massively underestimated the shortage of workers in the sector, because the modelling was a draft.

The forecast, obtained by AAP under freedom of information laws, shows an extra 35,000 aged care workers could be needed this financial year as Australia's population gets older and more people require support.

Despite pay rises aimed to attract new workers, nearly 120,000 more staff will be needed by 2035/36, including nurses, nurse practitioners and personal care workers, the note says.

Officials from the health and aged care department said the document relied on a model yet to be verified.

"That reporting referenced some internal draft outputs from a model and those internal draft outputs were not used in any fashion," first assistant secretary Emily Harper told senate estimates when asked about AAP's story.

Nowhere in the document released to AAP is it described as a draft.

Department boss Blair Comley suggested the memo shouldn't have been made public because it was a "working document".

Staff used the same reason to defend not telling the aged care minister about the forecasting error.

The flaw which the memo tried to fix relates to mandatory care minutes, a requirement that aged care residents receive at least 215 minutes of care every day.

The old model assumed once a person had received their mandated 215 minutes, staff could move on to help other residents.

In its updated analysis, the department conceded some older people might need help for longer.

The memo warned the change in approach created a "significant widening" of the workforce gap.

Officials also said they don't know how many older Australians were languishing in hospitals because they could not get into aged care.

A growing backlog of older people in hospitals has been the subject of numerous complaints from state governments, which argue their federal counterpart should do more to open up nursing home beds.

Asked how many people are impacted, Mr Comley said some governments collected information on the issue but more consistency was needed.

"We would not definitively know," he told the parliamentary inquiry.

States claim the lack of aged care spots is forcing them to spend millions of extra dollars to keep people in hospitals.

Commonwealth and state ministers are expected to discuss the issue at a meeting next Friday.

Opposition aged care spokeswoman Anne Ruston told the hearing the department had its "head in the sand" on the issue.