Labor's Workplace Relations spokesman Brendan O'Connor headed the group that included federal MPs Sharon Bird and Stephen Jones, and state MPs Ryan Park, Anna Watson and Paul Scully.

The letter took issue with the company's refusal to continue with the existing job security clause that stops workers from being replaced by contract labour.

"We do not support employers choosing the 'nuclear option' of terminating agreements, forcing workers back onto the award, undermining their pay and conditions," the letter read.

"We strongly request you to come back to the table, resume negotiations with your workers and simply agree to retain the existing job security clause."

READ MORE: Dispute one of the worst in the country

Negotiations have not stopped - PCKT and the CFMEU met in front of the Fair Work Commission last week and will do so again on Tuesday.

The letter noted that negotiations for a new enterprise agreement began in 2015 and, in that time the permanent jobs it covered have fallen from 107 to 51.

It also accused PKCT for a lack of compromise.

"The workers have negotiated with the company in good faith and made a number of concessions during the negotiating process," the letter read.

"We do not consider that PKCT has done the same thing."

On Monday PKCT locked out its workers for the fourth consecutive time.

Due to a combination of lockouts and strike action, the workers have not been inside the terminal since January 18.