UPDATED 2.51pm: The Prime Minister's being blasted for making excuses, after costs for a new child support system rocketed from $30 million to $163 million.

John Key says most IT projects go over budget, and the IRD was previously struggling with an outdated system designed in 1991.

Labour's revenue spokesperson Clayton Cosgrove says that's an excuse for not keeping an eye on the finances.

"There's over-budget and there's a budget blowout.

"So the question is $30 million, becomes $120 million, becomes $210 million then revised down to $163 million."

The child support reform affects 134,000 paying parents and 138,000 receiving parents. 

The change has been delayed till next month to allow Inland Revenue time to change its computer system.

The cost blowout has partly been caused by further changes to the enabling legislation introduced by Parliament last week.

If passed, they'll take the cost to $163 million.

In comparison the cost of merging the computer systems of Auckland's eight previous councils into one ended up being $157 million, which was in itself a blowout from an estimated $71 million.

Meanwhile, an expert says technology today can help drive down the costs of major projects despite the 

However the government's biggest schemes involving IT are blowing out way over budget.

Chief Executive of NZ Tech Graeme Muller says ironically, technology costs are getting cheaper.

"With projects, people have made an estimate then they go into more detail and they go, 'actually, if you want to do all those things it's going to be a lot more complex than we thought' and theoretically IT or technology can be used to help drive down those costs."