An attack ad is being rolled out ahead of five by-elections on July 28 and parliament debating the Turnbull government's tax cut draft laws this week.

In the television ad, the federal opposition accuses the prime minister of being more concerned about the top end of town than schools and hospitals.

If this second phase of the tax cut plan is passed, every company in Australia earning more than $50 million a year will benefit.

Mr Turnbull's pecuniary interest register shows through at least 15 of his 39 managed funds, he invests in 18 firms that have a turnover of more than $50 million.

A further 14 businesses have a turnover of more than $1 billion a year.

Labor says this shows the prime minister stands to profit from his company tax cut plan.

The government is still short of the numbers it needs to pass the company tax cuts this week but will negotiate with One Nation and the Centre Alliance to secure a win.

The debate comes as a new Australia Institute-ReachTel poll shows low levels of support for company tax cuts in two of the by-election seats.

In the South Australian seat of Mayo, only one in four voters back the cut, while in Queensland's Longman one in three voters are in favour.

Both electorates want the government to prioritise spending on services and infrastructure and reducing debt, before tax reductions.

The poll also showed a tied race in Longman between Labor and the Liberals in two-party terms.

And in Mayo, the Centre Alliance's Rebekha Sharkie had a clear 62-38 lead over the Liberals' Georgina Downer.

Australian Associated Press