More than eight staff members from the Chinese Embassy were spotted scouring Hill Head beach arriving in cars with official Chinese diplomatic plates and Westminster parking permits.
The group was spotted at 5pm Saturday, April 20.
Ian Paul, whose wife owns a beach hut at the site, was surprised to see the dignitaries arrive in BMW and Mercedes cars.
He said: 'They just trooped out onto the sand, and I thought - what the hell is going on? We were all watching them.
'The tide was so far out it seemed like they were going to walk to the Isle of Wight.'
Ian described residents watching each member of the group 'struggling' to carry two large buckets back to the cars, making repeated trips for two hours.
He said: 'They were harvesting the whole lot. It can't be right. A few oysters is fair enough, but two hours collecting them seems a bit much.'
Ian said that several residents saw oysters being collected.
The picking of oysters is restricted, with a closed season in effect from March 1 to October 31.
The Deputy Chief Conservation Officer at the Inshore Fisheries and Conservation Authority, Neil Richardson, said: 'Fisheries byelaws do not just apply to commercial operators.
'Any one collecting for personal consumption should familiarise themselves with these regulations as well as any local rules.'
A spokesperson for the organisation said they were investigating the matter, with conflicting reports on the types of shellfish having been picked.
A representative of the Chinese Embassy, in Marylebone, London, said: ‘We are not aware of any official delegation visiting Portsmouth and don’t think the matter mentioned is a regular occurrence.
'The Embassy nevertheless attaches importance to this information.
'We understand several staff did visit the beach and picked some shellfish for fun, being unaware of any forbidden notice.
'The Embassy has advised all staff who wish to visit beaches in Portsmouth and elsewhere to be aware of and strictly abide by the local regulations.'
The beach regularly attracts rule-abiding and rule-breaking shellfish pickers, according to the Hill Head Residents Association.
Association chairman Bill Hutchinson said: 'It's been a long-standing issue. Some times we get commercial outlets scouring the beach for shellfish.
'The high point was when we had 40 Chinese people down here. Some people do it because it is an important part of their culture, others do it for personal gain.'