There have been three resignations in Todd Barclay's Clutha-Southland electorate since the start of the year.
Mr Barclay (25), who grew up in Dipton and Gore, was elected to Parliament in 2014 after Bill English opted to become a list MP.
Mr Key told Radio New Zealand this morning that Mr Barclay had his full confidence.
"I've had the discussion with Bill [English] about it, because he was the former MP, but it's not something of major significance," he said.
"I mean there's clearly been a desire to have a change in staff and it's changed. It's not, as I said, entirely unusual when you get a new MP."
Mr Barclay was doing a good job, Mr Key said.
"He is a young man and I think he is doing a good job. It is always hard when you take over an electorate that has been occupied by such a successful, both local MP and National political figurehead as Bill English.
"But he is a young guy who is on a steep learning curve, and I'm sure he's going to do a great job for us."
Long-serving staff member Glenys Dickson left Mr Barclay's Gore office last month after 18 years in the job. The circumstances of her departure have been kept under wraps.
Electorate chairman Stuart Davie has also resigned, calling his position untenable. Queenstown electorate secretary Barbara Swan has also resigned, and is working out a notice period.
Prior to entering Parliament, Mr Barclay worked as corporate affairs manager at Phillip Morris for eight months. He also worked in National Government ministerial offices.
