MediaWorks has bought the two Marlborough Media Stations, Sounds FM and Easy FM, leaving only two significant permanent stations - 1XXFM Whakatane and Port FM Timaru - outside the MediaWorks-The Radio Network duopoly.
MediaWorks' radio division, RadioWorks, announced it had bought the Blenheim-based company, which broadcasts Sounds Radio into Blenheim, Picton and Kaikoura, for an undisclosed sum.
Marlborough Media - owned by former National MP Lynda Scott-Kelly - also operates Easy FM which broadcasts into Blenheim and Picton.
RadioWorks chief operating officer Sussan Turner said the company was interested in further acquisitions if owners were interested.
But according to David Innes of the commercial radio body, the Radio Broadcasters Association, apart from the Christian Radio Rhema, there were only two significant permanent commercial stations not owned by RadioWorks or TRN.
Radio 1XXFM in Whakatane is run by private radio veteran Glenn Smith and Port Radio in Timaru is owned by magazine publisher Reg Birchfield.
Innes said The Radio Network (50 per owned by New Zealand Herald publisher APN News & Media) has been securing temporary licences to broadcast in places it did not have a licence.
The expectation was that they could be tendered by the Ministry for Economic Development at a later date and it could bid if they bought in sufficient advertising revenue.
Innes said the process of acquisitions had been running a long time and was because of the benefits from sharing costs as part of a wider group.
TRN - which was built from the former Radio New Zealand commercial networks - had a wider spread of stations but required frequencies in Wellington and Christchurch.
The Marlborough purchase entrenches competition between the two radio companies with the local stations joining RadioWorks networks The Rock and The Edge beaming into the area.
MediaWorks also owns TV3 and C4.
TRN broadcasts Classic Hits and Newstalk FM with some local content.
Turner said the purchase of Marlborough Media gave instant presence in several important South Island markets, with two strong brands.
She said Sounds FM was especially well known throughout the region and there were no plans to convert it into a More FM station as had occurred in Gisborne in 2006.
She would not rule out Easy FM being aligned with its Easy Listening national network The Breeze.
Both MediaWorks/RadioWorks and TRN are going through a period of change this year.
MediaWorks had been a publicly listed company controlled by Canadian media company CanWest but was privatised by Australian private equity company Ironbridge Capital.
The Radio Network is 50 per cent owned by APN News & Media and 50 per cent by Clear Channel Communications in the United States, which is being restructured with new interest from private equity interests. Clear has indicated it is selling its radio interests outside the US.