Cancellations between October and December are the latest blow to Australians stranded overseas desperately hoping to return before the end of the year.
Singapore Airlines spokesman Karl Schubert said the company had not been told when international arrival caps would be lifted.
"We've had to make the very difficult decision that we simply can't operate the two additional flights that we were hoping to do into Sydney and to other ports across Australia," he told ABC radio on Wednesday.
The major airline has been unable to secure talks with the government.
"That has been frustrating at times, understanding that governments have had quite a lot of work to do and had their focus directed elsewhere," Mr Schubert said.
"What we need to do is get airlines, airports, governments at state and federal level around the same table to talk about how Australia is going to reopen."
A national reopening plan has raised hopes international travel can resume when 80 per cent of Australians aged 18 and above are fully vaccinated.
Under that phase, caps on returning Australians would be abolished and capacity for students, economic migrants and refugees increased.
Restrictions on Australians leaving the country would be lifted and new travel bubbles including with Singapore considered.
But Mr Schubert said the rules around what travel would look like were unclear.
"We just don't have the clarity we need to have the confidence to operate," he said.
Singapore Airlines is operating two flights a day across four Australian airports, with passenger services carrying between 12 and 25 people.
Mr Schubert said more information from the government would allow flights to be reinstated.
"We have the capacity to move very quickly to reinstate operations where demands warrants."