Officials from the prime minister's department have argued that releasing details about suggested guests for the White House dinner could impact US relations.
The mere revelation of whether Scott Morrison's staff suggested potential guests for the dinner in his honour at the White House could jeopardise relations with the United States, senators have heard.
Labor frontbencher Penny Wong has slammed officials from the prime minister's department for obfuscating on the question of whether Mr Morrison wanted Hillsong pastor Brian Houston invited.
While Mr Morrison was in Washington for the dinner and a series of events with US President Donald Trump, the Wall Street Journal reported the White House had rejected Mr Houston as a suggested guest.
The prime minister has repeatedly refused to answer questions about the matter, dismissing it as gossip.
Senator Wong quizzed senior officials from the prime minister's department on Monday whether his office had suggested any guests at all.
"I think any matters relating to the compilation or consideration of the guestlist for a state dinner provided by the US would be a matter that might impact on international relations," bureaucrat Gerard Martin replied.
Senator Wong: "You've got to be kidding. I'm not even asking you at this stage who's on it."
Mr Martin said the department had not provided any list of suggested guests.
But asked whether the prime minister's office had, he began his prepared answer again before Senator Wong cut him off to ask who he had discussed that response with.
He said he had discussed it with deputy secretary Stephanie Foster, but not department head - and former Morrison chief of staff - Phil Gaetjens.
Pressed again on why he wouldn't say whether the PMO provided a list, Mr Martin replied: "Senator, I'm afraid I've answered that question."
Finance Minister Mathias Cormann came to his rescue, firmly telling Senator Wong: "The official has got nothing else to add."
"What I'm prepared to do in an abundance of helpfulness, I'm going to take your interest in relation to this important matter in the bilateral relationship on notice and if there's anything else we can add to it on notice then we will, of course, do so," Senator Cormann said.
Senator Wong said anyone watching would understand what Mr Martin had been asked to do.
"I think it is entirely reasonable for Australians to know whether or not the prime minister asked for Mr Houston to go, given the concerns about Mr Houston's background," she said.
"And the fact that he has refused to answer this on so many occasions, that you have refused to answer this in a Senate chamber and here, and worse that public servants have been asked to not answer this question really says something about this prime minister's commitment to the truth."