A row over the removal of a health food ratings website has claimed the first scalp of the Abbott government and Labor is now gunning for Assistant Minister for Health Fiona Nash.

Senator Nash on Friday accepted “with regret” the resignation of her chief of staff Alastair Furnival, who has been embroiled in the row over the taking down of the website after just 20 hours of operation.

Mr Furnival has a shareholding in the lobbying firm Australian Public Affairs, operated by his wife Tracey Cain, which represents food industry opponents of the website including Kraft and Cadbury.

Labor had insisted there had been a serious breach of the ministerial staff code of conduct and opposition Senate leader Penny Wong accused Senator Nash of misleading parliament in her explanations over the website’s removal.

Mr Furnival resigned on Friday just hours after Senator Nash expressed confidence in him, telling reporters in Sydney he had done a “terrific job” and denying any conflict of interest.

But opposition health spokeswoman Catherine King said Mr Furnival’s resignation did not absolve Ms Nash.

She demanded the senator take responsibility for the apparent conflict of interest.

“The buck stops with Senator Nash,” Ms King said in a statement.

Senator Nash had to explain what she knew about the link between her staffer and the site, and the decision to delay the roll out of the Health Star Rating system, she said.

Senator Nash has said the site was taken down because a draft form had inadvertently gone up.

Mr Furnival’s resignation letter maintained he had not acted improperly.

“I regret any embarrassment that may have been caused to the minister and especially to my wife, who has been dragged into this political smear campaign,” he said in the letter, published on Fairfax Media, which criticised the “political attack” which distracted from the work of the health department.