Chief executive Paul Graham delivered the frank assessment as he pushed for politicians to "put the national interest first" and change the 1989 Australian Postal Corporation Act.
Mr Graham said rules forcing the service to deliver mail every weekday on time in 98.5 per cent of cases and having post offices close to households was not fit for purpose.
He also described regulation setting out the price of stamps as draconian, saying the rapid decline of mail meant it was now simply unsustainable.
"What we're asking for is that the goodwill of politicians is focused on doing what's right for Australia Post in the communities that we serve, not what is politically advantageous," Mr Graham told reporters in Melbourne on Thursday.
He also confirmed 400 head office jobs would go by June during an address to the American Chamber of Commerce in Australia function in Melbourne.
"The Australian community must understand that without change to their national postal service, its long-term viability is at risk," Mr Graham said in his speech.
The organisation previously announced it would post a financial loss this year after its letters business lost $189 million in six months.
"We want to keep Australia Post strong for all Australians and especially communities in rural and regional Australia, but this is only possible if we are financially sustainable," he added.
The address came on the same day as the public consultation period for a review into the future of the postal service closed.
Mr Graham said the organisation was at a crossroads and the status quo was no longer an option, with mail volumes dropping 66 per cent since the 2008-2009 global financial crisis.
The average household receives fewer than two letters per week and that is expected to halve within the next five years.
Mr Graham - who was speaking at a hotel in Southbank in central Melbourne - said there were 45 post offices within five kilometres of where he was standing.
"The general consensus would be it's probably too many, but we have different needs in metropolitan CBD areas than we have in regional and rural towns," he said.
There are more than 4000 post offices across Australia located within five kilometres of homes in metropolitan areas and seven and a half kilometres from people living in regional areas.
The Communications Workers Union opposes any cuts to Australia Post deliveries.
National president Shane Murphy said ensuring posties continue to deliver five days a week would allow Australia Post to grow its parcel services.
"We believe there is ample opportunity for Australia Post to grow and prosper, whilst allowing for its service obligations to our communities to continue," Mr Murphy said.
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