The Australian Taxation Office has admitted its working hours do not meet community expectations and are inefficient.

Key points: ATO staff have finished work at 4:51pm for many years despite management's reservations

ATO staff have finished work at 4:51pm for many years despite management's reservations Proposal would extend working hours to 5:00pm, an extra 4.5 working days per year

Proposal would extend working hours to 5:00pm, an extra 4.5 working days per year Internal ATO briefings note many staff work well beyond 4:51pm

Staff at the ATO have one of the shortest working weeks in Government but when they were asked to work an extra nine minutes a day to boost productivity, they responded with a backlash until the proposal was dropped.

Documents obtained by the ABC under freedom of information laws reveal the push to extend working hours to 5:00pm — an extra 4.5 working days a year — proved "highly contentious" and was ultimately dropped to ease concerns.

ATO staff have finished work at 4:51pm for many years despite management acknowledging the roster is out-of-step with community expectations and the rest of the bureaucracy.

The proposal would have improved productivity by 2 per cent and was made amid protracted workplace bargaining with unions that have now stretched into a third year.

"Of all the changes proposed in the enterprise agreement (EA) package, this was the one you told us you disliked the most," ATO briefing packs reveal.

"It was clear from your feedback that this had to go and I think it goes a long way to demonstrating that we're genuine about getting an EA in place for the next three years."

The largest union for Government workers — the Community and Public Sector Union (CPSU) — said the idea that ATO staff headed home en masse at 4:51pm was "ludicrous".

"Our members are working longer hours than ever, including unpaid overtime, because of over 4,000 jobs that have been slashed from the ATO in recent years," CPSU national secretary Nadine Flood said.

"This proposal was taken off the table well over a year ago.

"Clearly it wasn't the sticking point to reaching a new agreement because there have been two more emphatic 'no' votes from Tax Office staff since."

ATO staff have finished work at 4:51pm for many years. ( Supplied: ATO )

Staff at the ATO have now rejected three EA proposals — the latest in May and December 2016 with a 71 per cent voting majority — and have not had a pay rise since 2013.

They will not receive back pay.

Internal briefings show ATO management may have been caught off-guard by the level of backlash.

"The majority of feedback from our employees has indicated a willingness to work an additional nine minutes a day as they already work at or more than 7.5 hours and also — quite appropriately — to underpin a pay rise," documents said.

"It also satisfies the bargaining policy as a legitimate form of productivity to justify a pay rise as it is demonstrable, permanent and easily measurable.

"It results in an increased availability of our workforce."

The documents note that many staff work well beyond 4:51pm, particularly senior staff with more responsibilities.

Work hours do not meet 'community standards'

The documents show ATO management have known their working hours have not aligned "with broader community standards" for at least three years.

Despite this, unions campaigned for no increase in standard working hours.

Documents show ATO management have known their working hours have not aligned "with broader community standards". ( ABC News: Ian Cutmore )

The documents show the 2 per cent productivity increase would have underpinned a pay increase for staff, although this was capped at 1.5 per cent by then employment minister Eric Abetz.

"The move to a [7.5 hour] day brings the ATO in line with most other APS agencies and can be used as one of the biggest sources of genuine productivity to justify other improvements in the EA," the document said.

"An increase to the working day of nine minutes per person per day can generate significant productivity - which could be used to justify other increases in entitlements, such as a higher pay rise.

"However, it would be highly contentious with unions and employees, especially if there is not financial compensation."

Ms Flood said the nine-minute proposal was never the only issue for CPSU members, claiming there were 33 cuts to workplace conditions.

"They were understandably upset at a cut to their hourly pay rate via changing working hours at the same time as they were being told to accept a measly pay offer at that stage of 0.8 per cent a year," Ms Flood said.

"What's inefficient and out-of-step with community expectations is the Turnbull Government cutting thousands of jobs from the agency that polices multinational tax avoidance and demanding the staff, who are left [to] cover for that hole while spending three years pursuing a long list of cuts to their rights while sticking them on a wage freeze."

Labor raised concerns last year that more than 4,000 job losses at the tax office had limited its ability to investigate tax avoidance.

In a statement, an ATO spokeswoman said the office remained committed to reaching a good outcome for staff, the Government and taxpayers.

"This includes that we meet our service commitments to the community," she said.

"We are currently seeking feedback from staff to inform the EA process and ensure our engagement and communication approaches meet our employees' needs."