SANTIAGO, Oct 4 (Reuters) - Supervisors at Antofagasta Minerals' Los Pelambres copper mine in Chile expect to reach a new contract agreement in a mandatory mediation process with the company, which is set to begin later this week to avoid a strike at the site, a union head said on Tuesday.

Last week, 94% of union members voted in favor of a walk-off after failing to reach an agreement in the formal negotiation period.

Local legislation, however, requires a five-day government-led mediation, which can be extended to 10 days.

"Expectations are high. We maintain the demands we set out in the draft collective agreement and I think that in these five days, or even if it is extended, we will achieve a large part of our demands," the head of the union, Waldo Perez, told Reuters.

Mining supervisors are non-manual workers overseeing the mine's operations, including engineers, architects and logistic employees.

Perez said the talks would formally begin on Wednesday.

"We are pretty confident because it is a company that is financially healthy and also because of the results that the Los Pelambres mine brings to the (Antofagasta) group," he said.

In addition to the base salary adjustment and variable bonuses, the union leader noted that the most complicated issues during the negotiation process have been career development plans, which the group wants to include in the labour agreement.

Antofagasta Minerals declined to comment on the talks via media, but said it believes "that through dialogue we can reach a good agreement."

Earlier in the day, the company said it expected its copper output this year to come in the lower end of its estimated range of 640,000 to 660,000 tonnes after an accident at Los Pelambres, which produced 336,300 tonnes of copper in 2021. (Reporting by Fabian Andres Cambero Editing by Marguerita Choy)