Finance spokesman Jim Chalmers and communications spokesperson Michelle Rowland issued a statement asking Turnbull to "release advice about how his mishandling of the NBN rollout could smash the budget by blowing out deficits and pushing up debt even higher".
The pair cited an article in The Australian newspaper which claimed the Liberal Party had been told officially that the multi-technology mix NBN was exposing the budget to billions in writedowns.
On Monday, NBN Co chief executive Bill Morrow expressed fears that the network would never make any money.
Turnbull also criticised the network the same day, calling it a "calamitous train wreck" and blaming the Labor Party for the mess.
"No-one would've wanted to start from where Labor left us, so we have done the best we can getting that project on track," Turnbull said.
But Chalmers and Rowland said it was Turnbull who had "trashed the NBN business model so badly that he risks pushing record debt up even higher on his watch".
They said net debt was predicted to hit record highs over the next two years and gross debt had crashed through the half-a-trillion-dollar mark for the first time.
"Turnbull needs to immediately release the full advice so Australians can see just how bad his NBN bungles have jeopardised the budget," the Labor duo said.
"We also reiterate calls for the government to immediately endorse Recommendation 2 of the NBN Joint Standing Committee, which sought an independent audit of the assumptions underpinning the financial forecasts contained in the 2018 NBN Corporate Plan.
"If Turnbull really stands behind his multi-technology mix, then he needs to show (that) the numbers behind it stack up too."