After six months of dealing with Barnaby Joyce, tensions with China, and grumpy pro-coal backbenchers, the prime minister got his income tax plan through the Senate.

Turnbull can now take the cuts to three by-elections the Liberals are contesting in July as tangible evidence he's trying to put more money back in people's pockets.

The coalition took a hard tack through the negotiations - refusing to split off the controversial third stage of the bill that mainly helps people on big wages.

Labor couldn't support it, so now Turnbull gets to tell voters the opposition didn't want to give them tax cuts.

But Labor also has a solid attack line thanks to the third stage, which hands the biggest cuts to big earners.

The July 28 by-elections in Mayo (SA), Braddon (Tasmania), and Longman (Queensland) loom as a test of how well Turnbull's tax plan has cut through, and whether people are listening to Bill Shorten's plan for a bigger tax cut.

Labor is expected to win the two head-to-head contests it has with the coalition, but the size of the swing will be heavily scrutinised.

If Turnbull picks up one or more of the seats off the back of the tax cuts, expect lukewarm leadership mutterings within Labor to get steadily hotter.

Australian Associated Press