Australians wishing to buy shares in the stock market float of Medibank Private have until Friday to do so.
Shares must be applied and paid for by 2359 AEDT on that day.
Prospective shareholders in the health insurer can apply online, by post or through a broker.
Finance Minister Mathias Corman said retail investors submitting postal application forms with a cheque or money order payment needed to be mindful of mailing times.
People making electronic payments are also urged to be aware of earlier cut-off times.
The federal government has previously indicated that retail investors are likely to pay between $1.55 and $2 a share.
Medibank Private is due to list on the Australian Securities Exchange on November 25.
More than 750,000 people pre-registered to receive the Medibank Private prospectus in October.
Medibank was formed in 1976 and has been Australia's biggest health insurer since 1981.
The Howard government secured its privatisation in 2006 but its sale was delayed until the coalition was returned to power.
"The sale will also remove the current conflict where the government is both the regulator of the private health insurance market and owner of the largest market participant," Senator Cormann said in a statement.
Proceeds from the sale will go towards a $5 billion commonwealth government program that gives state governments incentive payments for selling assets to build infrastructure.
