A merged VHA, the two companies said, will maintain all existing mobile voice and data plans on offer from Vodafone and 3 for the next two years, even as it offers new deals to the market.
The offer is a last ditch ploy to convince the Australian Competition and Consumer Commission to approve the merger, which has been approved by Hutchison shareholders.
The ACCC's preliminary view on the merger, published some six weeks ago, was that "the proposed merger raises competition concerns in the short to medium term."
The regulator expressed concern that Vodafone and Hutchison are each other's direct competitors in that they both focus their services on "price-sensitive customers in metropolitan areas."
"The ACCC is concerned that the removal of Hutchison as a vigorous and effective competitor will lead to increased prices for customers," the regulator said.
The ACCC originally said it would make a final decision on the merger by May 6, but has since extended its options until this Friday.
The two mobile operators refuse to reveal how much of their combined customer base are currently out of contract.
It is these customers - for whom the carriers could change terms and conditions - that the two-year promise could be of benefit.
A spokeswoman for 3 argues that most on-contract customers could also benefit, as they would be able to negotiate new deals on old prices should their contract end within the next two years.