The ACTU has slammed the almost $5 million salary being paid to Australia Post chief Ahmed Fahour as the government-owned company prepares to axe 900 staff.

The blast from the nation's peak union body came amid a threat of industrial action in response to Australia Post's confirmation on Tuesday of massive job cuts, the majority of which will be made at its headquarters in Melbourne over the next 12 months.

ACTU secretary Dave Oliver on Wednesday said the salary paid to Mr Fahour, a former National Australia Bank chief, is particularly extraordinary at a time when there are calls from business leaders and other high earners to lower wages and even drop the minimum wage.

"Something is wrong when Australia Post chief Ahmed Fahour is paid millions a year but 900 workers, possibly more to come, are losing their jobs because of lack of profitability," Mr Oliver told AAP on Wednesday.

Mr Oliver said CEO pay had increased eight per cent over the past year, while the ACTU says the chief of a top 50 company earns almost 100 times more than the average worker.

"These mind bogglingly large pay packets make the attacks on the pay and conditions of ordinary Australians even more unfair," he said.

Mr Fahour was paid $4.8 million last year for his dual role as chief executive officer and managing director of Australia Post, a company which is 100 per cent owned by taxpayers.

In comparison, Royal Mail chief executive Moya Greene, who last year oversaw the privatisation of 60 per cent of Britain's postal company, was paid a $2.23 million, while US Postal Service boss Patrick Donahoe was paid about $595,000.

The British and US postal services easily dwarf Australia Post.

Mr Fahour on Wednesday dodged questions about his pay, saying "compensation is a matter for the board" as he defended the job cuts as necessary if Australia Post was to remain sustainable.

The cuts reflected accelerating falls in demand in letter deliveries that will, in the next five to 15 years, result in 99.99 per cent of mail being received electronically, he said.

"The rate of decline in letters is accelerating at such a pace we do need to reform those services," Mr Fahour said.

He also confirmed that Australia Post would seek legislative changes that would allow it to reduce standard mail deliveries from five days a week.

Jim Metcher, from the Communications Electrical and Plumbing Union, said strike action was a possibility, as he called for a meeting with Mr Fahour to discuss the job cuts in detail.

"We don't rule out anything in terms of any campaigns to be conducted by the union, involving the workforce against Australia Post," Mr Metcher told AAP.

He said his union would accept changes only where they were proven "justified and necessary".

The Community and Public Sector Union, which represents the managers, executive assistants, sales and marketing staff, human resource and IT staff in line to lose their jobs, will meet with Australia Post management on Thursday before deciding whether to take industrial action.