The Government's plans to clamp down on zero hour contracts and extend paid parental leave hang in the balance after its support partners told Workplace Relations Minister Michael Woodhouse they won't support it unless serious changes are made.

The Employment Standards Bill is due back before parliament this week – and needs to be passed in the next few weeks so the extension to 18 weeks paid parental leave can come into force on April 1, as promised.

Unions and opposition parties last year put pressure on Mr Woodhouse to scrap the so called zero hour contracts.

And while Mr Woodhouse had promised to abolish the worst parts of those contracts, there are two main sticking points on the bill that mean that as it stands the Government doesn't have the numbers to pass it.

So he's scrambling to get support – and has been locked away in talks with MPs from Labour, United Future and the Maori Party.

MPs from those parties have confirmed to ONE News that unless changes are made, they won't support it.

The two main issues are around the lack of guaranteed hours for workers and what's called the "availability clause" – which is when people are told to be available for work but not necessarily given that work.

Under the bill, they would have to be paid "reasonable compensation" if they have that clause in their contract, but there's no definition of compensation.

Unions and Labour say if that's not defined, it'll end up with courts having to define reasonable compensation.

ONE News understands the changes negotiated between Mr Woodhouse and other MPs will "improve" those issues – improving them enough to support the bill.

That means there'll be pressure on employers to provide guaranteed hours and people only being given availability clauses for hours they do over and above their usual hours.