A third Labor candidate has broken ranks to back calls for the party to ditch fossil fuel subsidies and ban political donations from mining companies.

Lisa Singh, Tasmanian senator and shadow parliamentary secretary for the environment, has signed the "pollution free politics pledge", forcing Opposition Leader Bill Shorten to restate that Labor was not considering ending the diesel fuel rebate available to miners and farmers.

Tasmanian senator Lisa Singh, pictured during an estimates hearing at Parliament House, has signed the "pollution free politics pledge" being pushed by green group 350.org. Credit:Alex Ellinghausen

Fairfax Media revealed last week that Justine Elliot, the Labor member for Richmond, and Janelle Saffin, the former Page MP who is trying to win back the North Coast NSW seat, also defied party policy to sign the pledge being pushed by green group 350.org.

Senator Singh, a leading figure of the Left who was relegated to the unsafe sixth spot on Labor's Senate ticket in Tasmania by factional bosses, said Labor's 50 per cent target on electricity generation coming from renewables by 2030 required a transition away from $2.2 billion in fossil fuels subsidies.