Having spent $12.7 million on advertising in a bid to calm Sydney commuters frustrated with infrastructure construction works, the NSW government has now paid another $423,000 to track its effectiveness.

Meanwhile, documents obtained by the state opposition suggest the Tomorrow's Sydney ad campaign will be extended to the 2018-19 state election year.

The ads were launched in mid-2015 as an information campaign about expected disruptions from projects including the $2.1 billion CBD and south east light rail, $12 billion Sydney Metro and $16.8 billion WestConnex motorway.

In January Fairfax Media reported that a further $5.3 million would be spent extending the campaign to the end of October, bringing the total cost to $11.8 million.