Labor has ambushed the Coalition in Parliament, which has resulted in the government voting against its own key budget measures that it has been calling on the opposition to pass as a matter of urgency.

Opposition Leader Bill Shorten caught the Coalition by surprise by calling on a vote at the end of a speech on Wednesday morning, given the opposition supports the plan to let small businesses write off new assets worth up to $20,000 in one financial year instead of four.

The government voted against the bill, despite devoting a portion of question time this week urging Labor to pass the measure.

"Let us pass this bill straight away," Prime Minister Tony Abbott said on Monday.