The tenure of New Zealand’s opposition leader, Simon Bridges, appears to be hanging by a thread after he was accused by a fellow MP and former chief whip of concealing electoral donations and fabricating sexual harassment allegations.

In an extraordinary media conference that lasted nearly an hour, the National party’s Jami-Lee Ross said Bridges was a “corrupt” politician who was unfit to ever be prime minister. He accused him of concealing a NZ$100,000 (£50,000) party donation and the identity of donors as well as fabricating sexual harassment claims against Ross.

“Simon is a flawed individual without a moral compass and without any underlying principles except power,” said Ross. “He is a corrupt politician.”

Ross then promptly resigned.

Tuesday’s tensions are the culmination of a hunt by Bridges to uncover the identity of a National party MP who leaked information on his travel expenses to a journalist.

Bridges spent more than NZ$100,000 of taxpayers money on a nation-wide tour of the country, NZ$80,000 of which was spent on travelling in a crown limousine.

Bridges initially cast suspicion on the Labour party and speaker Trevor Mallard, but a PWC investigation released this week concluded that Ross was the most likely culprit, a finding Ross continues to deny.

Bridges pushed ahead with his investigation despite a plea to protect the mental health of the leaker.

The National party has enjoyed 10 years in government under the hugely popular John Key but is still struggling to recover from the 2017 general election result that saw Labour’s Jacinda Ardern catapulted into power.

Former prime minister Bill English resigned shortly after the election and, since being elected to lead the party in February, Bridges has struggled to win public favour, and has frequently been accused of elitism and being unlikeable, as well as suffering poorly in internal polling.

Asked for comment by reporters outside parliament deputy prime minister Winston Peters played the song “Burning Bridges” before bursting into gleeful laughter.

On Tuesday, Ross alleged Bridges had asked him to conceal a party donation of NZ$100,000 made by a wealthy Chinese businessman in May by splitting it into smaller amounts, and listing it under different names. He further accused Bridges of concealing the true identity of a donor who was a close friend.

Ross said he had photographs and a phone recording that would prove his allegations were true, and intended to take that evidence to police on Wednesday and file a complaint.

Ross also accused Bridges of fabricating four sexual harassment claims against him as part of a targeted smear campaign to push him out of the party.

Ross denied he had ever acted inappropriately around women during his parliamentary career and said the stress of the allegations caused him to suffer a “mental breakdown”, which worsened when Bridges told journalists Ross was suffering an “embarrassing” health condition.

Ross said he had asked Bridges for details of the harassment claims made against him but Bridges had advised him to take a leave of absence and that the four allegations could “easily” turn into 15.

Bridges comprehensively denied all of Ross’s allegations, and slammed him as a lone wolf who was “lying, leaking and lashing out”.

The allegations of concealing electoral party donations were “baseless” and “entirely false”, Bridges said, and the matter should now be investigated by police.

Bridges said he retained the strong support of his party and was flanked by the main likely contenders in any leadership race, including Judith Collins, Amy Adams, Mark Mitchell and Paula Bennett.

Ross resigned from the National party, forcing a byelection, and said he would stand as an independent in his electorate of Botany. At a National party meeting held during Ross’s lengthy media conference Bridges said National MPs had unanimously voted to expel Ross from caucus.

A leadership poll conducted by Newshub on Monday found national party veteran Judith Collins was the strongest contender to take over as opposition leader.