Well the tip of the iceburg, I'll start to list some of the businesses that have surfaced so far.
For the majority I have found an advertisement and even a photo, plus referenced sources for further information.
Further information is available to peruse at The Treasury - Pre-1930 Thames Business Register Folders
Business Name, Type of Business, Known Location, extra information
ACADEMY OF MUSIC Theatre Brown Street
ADAMS E F Engineer Queen Street
ADAMS & HARDING Mining Agent Queen Street
ADLAM W J Butcher Pollen Street
ADVERTISER NEWSPAPER Newspaper Albert Street
AH CHAN Vineyards Totara Totara Vineyards (SYC)
AH NON Market Gardens Parawai
AIREY M M Clothier & Grocer Pollen Street
AKEROYD Spedding Butcher Brown Street, Grahamstown
ALGREN J F Jeweller Brown Street
ALLAWAYS Boarding house See: ALLOWAYS
ALLOWAYS Boarding house
AMERICAN THEATRE Theatre Shortland aka: Butt's American Theatre
ANTONELLI Frederick Music Teachers Thames
ARBURY J W Draper & Clothier Pollen Street
ARBURY & BERRY Butcher Cnr Richmond & Pollen Streets
ARDERN Undertaker
ARDERN Mrs Bookseller Pollen Street Late: Mrs Jefferson's
AUCKLAND & THAMES STEAM NAVIGATION COMPANY Transport Wharf
AUCKLAND STAR AGENCY
AVENELL W Butcher Pollen Street
AVERY Grocer Shortland
BAGGOTT James Sharebrokers Davey Street
BAGNALL L & J Coal & Wood Yard Davey Street See also: Hauraki Sawmills
BAIRD A S & Co Produce Merchant Albert Street
BAKER W B Carrier Pollen Street
BANE BROS Cabinetmaker Corner Mary & Pollen Streets
BANK OF AUSTRALIA Bank Albert Street see: Union Bank of Australia
BANK OF NEW SOUTH WALES Bank Albert Street, Pollen Street
BANK OF NEW ZEALAND Bank "Corner Brown & Albert Streets, Pollen Street
BANKS W Bakery Queen Street
BANKS & CO Stock Agent Parawai Thames Cattle Market
BARLOW'S RIFLE GALLERY Rifle Range Brown Street
BARLOW W J Painter Brown Street See: Barlow's Rifle Range
BARNETTs DINING ROOM Restaurant Pollen Street
BARNETT & LEVY Grocer Owen Street
BASSTIAN Charles Chiropodist Royal Hotel
BATEMAN Robt Butcher Rolleston Street
BATES J Clothing Pollen Street "The Elite Dress House"
BATES William Timber Merchants Shortland & Tookeys Flat
BATTSON Plumber Pollen Street
BAXTER Agnes Thames see: Baxter & Mathieson
BAXTER & MATHESON Storekeeper Golden Crown Street
BAYLDON Capt T C Harbourmaster Thames see; Harbour Board Thames
BEETHAM, WALKER & CO Surveyor Pollen Street
BEERE D M Engineer Shortland
BEERE E Holroyd Surveyor Shortland
BENDIGO STORE General Store Pollen Street
BENFIELD Mrs General Store Pollen Street
BINNEY E Auctioneer Albert Street
BINNEY Edwin Insurance Agent Grahamstown
BIRD & THOMSON Produce Merchant Rolleston Street Rolleston Street Co-operative Stores
BLOTT G Butcher Owen Street
BOBBETT William Butcher Rolleston Street
BONGARDS Chemist Pollen Street Shops North & South Pollen Street
BOON E G Draper & Clothier Cnr Pollen & Mary Sts
BOOTH T B Picture Framer Pollen Street
BOROUGH BAKERY Bakery Beach Road See: John Robinson
BOROUGH COUNCIL OFFICES Corner Albert & Queen Streets Pollen Street, Mackay Street Now part of Thames Coromandel District Council
BRADLEY Thomas & Co Livery Stables Thames Cobb & Co
BREARLEY M Butcher Brown Street
BREWER, MARTIN & BEERE WD, WE & EH Land Agents Shortland
BRIDGER T Clothier Albert Street
BRIERLY H Wheelwright Kuranui
BRITON & CO Timber Merchants Burke Street
BROAD & HAUGHTON Land Agents Pollen Street
BROKENSHIRE A J Bookseller & Stationer Pollen Street
BROKENSHIRE W & Son Hairdresser & Tobacconist Pollen Street
BROKENSHIRE & ROSS Plumber Pollen Street, Richmond St
BROWN C & ?Son ? Pollen Street
BROWN Elizabeth Restaurant Opposite Wharf Grahamstown See: Diggers Restaurant
BROWN & CO Isaac Cycle dealer Pollen Street
BROWN R Fruiterer Brown Street
BROWN T Horse Breaker Pollen Street
BROWNE M & CO Michael Grain & Produce Merchants Albert Street
BROWNING John Bakery Brown Street
BRUCE A Watchmaker Albert Street
BRUCE Albert Estate Agent Cnr Albert & Brown St
BRYAN A G T Solicitors Pollen Street See: Solicitors & Barristers
BRYAN & MACKLOW Solicitors Pollen Street See: Solicitors & Barristers
BUCHAN James Blacksmith Pollen & Queen Streets Late: G S Worthington
BUCHANAN & PURNELL Solicitors Thames See: Solicitors & Barristers
BUCKLAND Eliza Elizabeth Storekeeper Cnr Beach Rd & Burke Street
BUCKLEY Alfred & Joseph Builder Thames
BUCKLEY Roland Saddler & Bootmaker Brown Street
BUICK & CO Watchmaker Pollen Street
BULL & STURROCK Iron Merchants Albert Street
BULLOTT & CREAMER Grocer Pollen St aka Creamer & Bullot
BUNTING Chas Electrician Pollen Street
BURNS Archd Grocer Albert Street
BURRA Robert Bookseller Brown Street
BURT Harry Waterman Shortland
BUTEMENT BROS Aerated Water Pollen Street
BUTT John Agent Wharf See: Auckland & Thames Steam Navigation Co
BUTT'S AMERICAN THEATRE Theatre Shortland see: American Theatre
BUTTLE G A Sharebrokers Thames See: Sharebrokers
If your ancestors were connected to any of these businesses and you have any photos/information to share, please let me know.