Labor MP Craig Thomson cannot be guaranteed a parliamentary "pair" to attend the birth of his child if it occurs during the period of the carbon tax debate.

"We have made it crystal clear that only in the most extraordinary circumstances will pairs be offered for the carbon tax vote," Mr Abbott told reporters during a visit to Tumbi Umbi in Mr Thomson's NSW central coast electorate of Dobell.

Tony Abbott ... "crystal clear". Credit:Andrew Meares

"This is by far the most important vote that the Australian Parliament will take in the current term of Parliament and the first duty of members of Parliament is to be in the Parliament when critical votes are taken."

Mr Thomson is under scrutiny over the alleged misuse of a credit card when he was boss of the Health Services Union.