The key documents Assistant Health Minister Fiona Nash claims prevented any conflict of interest for a junk food lobbyist working in her office "do not exist'', according to government agencies.

Labor has called on Prime Minister Tony Abbott to sack the beleaguered senator over the food star-ratings scandal, which erupted when a website that allowed companies to voluntarily sign up to a healthy food star-rating system was taken down at the insistence of Alastair Furnival, a junk food lobbyist who was working as Senator Nash's chief of staff.

Ministers to discuss food star-rating system next week: Assistant Health Minister Fiona Nash. Credit:Alex Ellinghausen

Next week, state and territory food ministers will discuss the food star-rating system. It is understood it is likely the website will be re-instated, more than five months after it was taken down, despite having already been agreed to by the states.

Fairfax Media has been attempting to get hold of a letter Senator Nash said Mr Furnival wrote outlining how he would ensure his co-ownership of the lobbying company Australian Public Affairs would not interfere with his role in creating healthy food policy.