Collins' explanation at odds with Chinese firm's formal invitation, says Robertson.
Labour MP Grant Robertson says Judith Collins' claim that she dropped in to Oravida in Shanghai for a casual cup of tea does not ring true after a formal invitation from the company two weeks before the Justice Minister's trip was released in documents under the Official Information Act.
Mr Robertson has accused Ms Collins of a conflict of interest because her husband is a director of the milk exporting company and it put on its website a photo of her saying she had tasted and "endorsed" its milk.
Ms Collins has said she did not endorse the company's products and had made it clear in advance that it should not use her photo to promote itself.
Early last week, Ms Collins described the visit to Oravida in Shanghai as "a cup of tea on the way to the airport" and as a "20 minute cup of tea, with milk".
However, Mr Robertson said the written invitation showed the visit was "a well-organised publicity stunt and photo op, yet Judith Collins has spent the week trying to minimise and diminish it as a casual glass of milk".
The invitation was a letter from Oravida's chairman, Stone Shi. It referred to her pending trip to China and said Oravida would be "honoured for you to visit us in Shanghai on the 23rd October to witness our development, and our commitment to branding and building a reputation for New Zealand products".
Ms Collins' office released the documents, originally requested by NZ First, to the Herald last week.
A spokeswoman for the minister said the written invitation followed up on an oral one extended to Ms Collins while she was at the opening of Oravida's new offices in Auckland. It was sent a few days later to ensure Ms Collins' staff had a record of it.
The spokeswoman said it was made clear that the minister could drop in only if she had a gap in her schedule. She had found the time to do so while on her way to the airport.
The documents released under the Official Information Act also show Ms Collins did not mention the Oravida visit in her report back to the Cabinet on the trip, despite mentioning similar visits to companies with New Zealand links in her report on an earlier trip to China in June 2012.
Ms Collins' spokeswoman said that was because it was a quick visit, and the minister had also not mentioned visiting a NZ coffee shop, Flat White, in China on the same trip.
The Prime Minister's office is refusing to release Cabinet Office advice that PM John Key says has cleared her of any breach of Cabinet rules.
Ms Collins has said it would not stop her returning to visit Oravida and it was "ridiculous" to suggest she avoid a company simply because a family member was connected to it.