The Pakatan Rakyat (PR) opposition pact machinery will continue to focus on the Goods and Services Tax (GST) during the Permatang Pauh by-election campaign, says PKR election director Datuk Saifuddin Nasution Ismail.

He said the decision was made based on feedback from the party's 28 polling district committees which revealed that the newly-implemented tax system was the main issue for the Permatang Pauh electorate.

"The GST is the top issue in this campaign, we are going house-to-house, and to markets and gathering places to distribute leaflets (on the GST)," he said at a joint daily press conference on the by-election at the Yayasan Aman here today, together with PKR vice-president Nurul Izzah Anwar.

Meanwhile, Nurul Izzah has asked the Home Ministry and the Prisons Department to allow her father, jailed Opposition Leader Datuk Seri Anwar Ibrahim to cast his vote in the by-election.

She said this was in accordance with Home Minister Datuk Seri Dr Ahmad Zahid Hamidi's recommendation in July 2013, to create a mechanism to allow prisoners to vote.

"I welcome the recommendation and suggest the Permatang Pauh by-election be made the pilot project, by allowing my father to vote on May 7," she said.

The Permatang Pauh parliamentary by-election was called after incumbent MP Anwar was disqualified as an elected representative when a petition of royal pardon against his imprisonment filed by his family was rejected by the Pardons Board.

Anwar is currently serving a five-year imprisonment at the Sungai Buloh Prison in Selangor for sodomising his former aide, Mohd Saiful Bukhari Azlan.

PKR president Datuk Seri Dr Wan Azizah Wan Ismail is contesting the by-election against Barisan Nasional candidate Suhaimi Sabudin, Parti Rakyat Malaysia's Azman Shah and Independent candidate Salleh Ishak.

The Election Commission fixed polling day on May 7.