A union has estimated as many as 10 air safety investigators could lose their jobs from the Australian Transport Safety Bureau, the air safety watchdog which has told Malaysia it is on standby to help if needed during its missing jet crisis.

The opposition is calling on Prime Minister Tony Abbott to protect transport safety from budget cuts in the federal budget.

The bureau has not yet been needed by Malaysia because the aircraft is still missing, although it has offered the support of its analysts if the aircraft is found.

David Smith, an official from the union representing ATSB staff, Professionals Australia, said the work of air, sea and rail investigators, mostly based in Canberra, had increased in recent years, partly due to its workforce of investigators dropping from about 60 to 48 through natural attrition. A further cut to the number of investigators of 15 per cent to 20 per cent would reduce the number of investigators by seven to 10 and take the total to about 40.

''We're willing to throw resources at the Malaysian Airlines issue but at the same time we're cutting back,'' Mr Smith said.

''I sought a commitment [from management] that would keep investigators and analysts out of the workforce cuts because that's the core function of the organisation. They've responded that they can't do that.'' He said the bureau was separated from the Infrastructure Department a number of years ago which had only led to the corporate side of the organisation, including administration, costing more.

Opposition infrastructure spokesman Anthony Albanese said the public needed to know Australia had the highest transport safety standards and retained a capacity to investigate accidents and near misses. ''Public safety should be any government's first responsibility. This vital work of the ATSB cannot be compromised in the search for spending cuts for their own sake.

''Tony Abbott must guarantee there will be no reduction in funding for the Australian Transport Safety Bureau in the budget in May.''

A bureau spokesman said funding will decrease by $2 million, or about 10 per cent, between 2013-14 and 2014-15. ''Staff reductions will be broadly in proportion to the reduction in revenue,'' he said.

''The ATSB Commission is satisfied that the reductions can be managed without significant impact on the ATSB's role in maintaining and improving transport safety.''

The spokesman said revenue from state governments for rail investigation work will be less than originally estimated and there has been a cumulative effect from efficiency dividends and savings measures.

''The ATSB's initial strategy was to manage the necessary staffing reduction through staff turnover and a recruitment freeze. It is now clear that further, targeted redundancies will be needed to maintain financial sustainability. The ATSB is developing a strategy to minimise the risk to ongoing safety investigation functions, including through improved selection and management of investigations and streamlined business systems and procedures.''