I think the following papers will help you answering your questions:

Toward understanding the role of auditing in the public sector

WR Baber - Journal of Accounting and Economics, 1983 - Elsevier

Internal audit effectiveness: an Ethiopian public sector case study

D Getie Mihret… - Managerial Auditing …, 2007 - emeraldinsight.com

Internal audit is an important function of the public sector organization, where tax payers money has been invested by the government. The internal audit is essential to ascertain that established norms and standards are being followed in organizational functions, and to find out deviation or exception if any, and bring the same to the notice of management, so as to enable the taking of corrective measures wherever necessary.This  is a value added function, as it enables management to identify wastage and loopholes in various functional areas of the organization and to take corrective measures, resulting in the increasing of stakeholders value creation across the organizational value chain.  

Internal audit is a very important mechanism of internal corporate governance. If you are interested in corporate governance, you may check my relevant papers. Some of them talk about the relationship between audit and governance. 

Internal audit in the Philippine setting follow strictly the Philippine Procurement Plan where a set of rules and regulations have to be followed.  Basically, an audit is performed to examine if all disbursements are within the approved budget by the Congress of the Philippines.

PSUs have been assigned an important role of socio-economical development of the country, Same time most of the PSUs are centre of corruptions, Hence internal auditor/s ensures three things

1. Money (shown in the books of accounts) have been disabused legally for the purpose to which it is applied.

2. Expenditure was authorised.

3. Proper apportionment / distribution of funds. 

Internal audit It is an instrument of Accountability. But it is equally important to the Govt. and management to improve its efficiency and effectiveness. This is achieved by bringing out financial and operational deficiencies, inadequacy of the system, shortfall in the systems etc, and by analysing the reasons for shortfall from acceptable standards of performance.

I think International Organisation of Supreme Audit Institutions (INTOSAI) is a good way.

http://www.intosai.org