A leading anti-gambling advocate fears there will be "more predatory" behaviour targeting problem gamblers in the wake of the $34 million takeover of TomWaterhouse.com.
The chair of the Australian Churches Gambling Taskforce, Tim Costello, fears there may be some nasty fallout from William Hill's .
The takeover gives the UK betting giant access to Tom Waterhouse's client list, including the betting habits of vulnerable gamblers.
"The buyer can target who are the losers, who are the winners," Reverend Costello said.
"Offer great deals to losers, bad deals for winners which is potentially an invasion of privacy, more predatory behaviour and unacceptable."
The editor of horseracingaustralia.info, Gary Robinson, says manipulating the client list is where the bookmakers cash in.
"They’ve got a complete list of all the losing punters of TomWaterhouse.com," Mr Robinson said.
"So they can concentrate their efforts on trying to get those people to punt more and obviously lose more money."
William Hill's acquisition of Tom Waterhouse broadens the British betting giant's online gambling stable in Australia that already includes Centrebet and Sportingbet.
Mr Robinson says betting companies keep a record of every transaction. They know exactly who loses and they know how to entice those losing punters to keep on betting.
"So they might offer them special prices on a unique set of bets so that it looks like value but it may not be," Mr Robinson said.
"For example in one of the instances they might have in a sport a first try-scorer market.
"So the market might come up and allow you to back your selected try-scorer and if they don't end up being a first try-scorer but get a try later on in the match, you get your money back.
"So this encourages you to possibly take more of those types of bets that are notoriously hard to win on."
Reverend Costello is uneasy about William Hill's growing list of clients in Australia.
"This is deeply disturbing for the public and very worrying that William Hill has this access," Reverend Costello said.
Mr Robinson says the takeover is also bad news for his clients who are professional or semi-professional punters.
He says William Hill can now identify successful punters and offer them lower odds to discourage them from laying bets.
Mr Robinson says there is also the chance that William Hill will not stop at TomWaterhouse.com and will continue to buy up other Australian betting agencies in a bid to continue to grow its sport betting business.