"A tombstone for a once great party that once did great things for New Zealand."
If successful in his bid, Mr Parker said he would review all of Labour's policies.
"We lost badly and I get it."
Mr Parker said Labour needed to get its house in order.
"History teaches us a house divided against itself cannot stand.
"All of us claim to be able to unite the caucus and the party.
"From unity comes strength. Unity also brings confidence - and success.
"I believe I am that leader."
His announcement brings the number of candidates for the job to four - David Cunliffe, Grant Robertson, Andrew Little and Mr Parker.
Mr Robertson had hoped Mr Parker would support Mr Robertson's bid for the leadership -- he had intended to keep Mr Parker in the finance role.
"I would have liked the support of David Parker for my first preference, but obviously I respect his decision to run."
He said Mr Parker's entry meant there was little chance of a clear winner from the first round of votes. Labour has a preferential voting system under which the losing candidate in each round drops out and their supporters' second choices are counted instead.
"This makes it a more crowded field and it's quite certain now that second preferences will become very important."
He would continue to focus on trying to secure votes of MPs, members and union affiliates -- or to get the second votes of those who were backing others, including, he hoped, Mr Parker's.
Read more:
• Editorial: It's time for Cunliffe to step aside
• Bryce Edwards: Political roundup: Andrew Little on track to be Labour leader
• Little banking on a clean break
• Labour leadership hopefuls get their bids in
Mr Parker is highly respected within Labour and his supporters believe he is best placed to ease the tension between caucus and members, unify caucus and get Labour back in front of the voters' eyes for the right reasons. From Dunedin but based in Auckland he also adds another non-Wellington name to that of Mr Cunliffe, as well as a business background.
He was appointed acting leader by caucus a week after the election after Mr Cunliffe stepped down to trigger the leadership contest.
Mr Parker's entry will make it almost inevitable that the contest will not be decided on the first round of votes.
Labour has a preferential voting system for its leadership under which members, caucus and affiliates rank candidates in order of preference.
In each round the lowest ranked candidate is knocked out and their supporters' second preference votes are tallied up until one candidate has more than 50 per cent.
- additional reporting Claire Trevett