SYDNEY, March 25 (Reuters) - Hundreds of staff at the Australian Broadcasting Corporation began a one-day strike on Wednesday, the first walkout in 20 years, over pay and working conditions, disrupting live news coverage at the public broadcaster.

Live programming was diverted to content from the UK broadcaster BBC when the strike began at 11:00 a.m. (0000 GMT).

Flagship programmes on TV and radio were also expected to be replaced by reruns during the 24-hour strike.

It comes after months of pay negotiations between staff and management. The majority of ABC staff this week rejected the latest offer of a 10% pay rise over three years and a A$1,000 ($700) bonus for ongoing and fixed-term staff.

One-third of the ABC's 4,500 staff are unionised. Around 1,000 people voted in favour of taking industrial action after talks failed.

The Media, Entertainment & Arts Alliance union said the offer was below inflation and failed to ensure secure work conditions. The A$1,000 bonus also excluded casual staff members, the union said.

"ABC staff are taking this step because they want fair pay that keeps up with the cost of living, genuine job security, and working conditions that allow them to continue serving the Australian public with integrity," Chief Executive Erin Madeley said. 

ABC Managing Director Hugh Marks said the strike was "very unfortunate".

"It is not a great time for our teams to be out. There is a lot of things happening in the world," Marks told ABC Radio.

"We will be using BBC content where that's appropriate and where that's available to us. We will be maintaining services but they won't be of the standard that I would like to be on air."

He said the offer given to staff was "fair and reasonable" and above inflation when the bonus was counted.

($1 = 1.4298 Australian dollars)