Pre-registration for shares in Mighty River Power will open tomorrow, the Government has announced.

"My expectation is that, subject to market conditions, this process will be completed in mid-May, most likely before the budget," Prime Minister John  Key said.

The budget is May 16.

The offer period was expected to open in mid-April and run for three weeks and to buy shares Kiwis would have to invest at least $1000. They would be guaranteed at least up to $2000 worth of shares.

There would be a loyalty bonus for New Zealand retail investors who held their shares for a minimum period, although the terms of this were not announced.

Finance Minister Bill English and State Owned Enterprise Minister Tony Ryall will travel to Auckland tomorrow to launch the pre-registration programme. People would be able to call an 0800 number and leave their details, with a prospectus then sent out.

This did not constitute a requirement to invest money, English said.

Mighty River Power was expected to have a secondary listing on the ASX.

English said there was nothing unusual about Mighty River having an ASX listing, with eight out of 10 of New Zealand's largest listed companies listed there.

Mighty River Power was always going to be first off the block and the way was cleared when the Supreme Court turned down a Maori Council appeal on water-ownership.

The company last month reported a net profit for the six months to December 31, of $75.5 million, up from $17.6m a year earlier.

- © Fairfax NZ News