"It's a bit of a struggle for [farmers] at the best of times.
"It's not just the cost of the stock, you lose the lambs as well."
He said the thieves must have been very organised and would have needed suitable transport and portable pens to have removed the sheep.
"You can't just walk off with 300 sheep unless you are organised. It's not like they carried them out on their back."
Mr McLellan said the police had been notified of the theft.
Alexandra police response manager Senior Sergeant Ian Kerrisk said in a statement "Central Otago Police are aware of local postings on social media about 'sheep rustling' but have not to date received any complaints for thefts of this nature."
Snr Sgt Kerrisk said this type of theft was unusual in Central Otago but any theft of stock was a concerning matter for the police.
When asked if they would follow up on the incident with Mr McLellan, police said they could not confirm whether they were talking to an individual complainant, but said they were "aware and are taking the matter seriously".
• Police urge anyone who has information about stock thefts to either contact their local police or call Crimestoppers anonymously on 0800-555-111.